What would be the safest, not-cheesy way to level grind in a sand-box like setting?

[Quertus]

When high level characters can cast Genesis to make new land for living space and create items to supply endless food, the amount of time between being born and casting Genesis becomes relevant for determining their population's exponential growth rate. If the time is significantly shorter for shorter-lived races, then their exponential takeoff will inevitably pull ahead.

Um... no? Say there's just 2 of one race, "Adam" and "Eve". Adam's build can Survival to feed 2 people, Eve 1. Even at level 999,999,999,999. They can support at most 1 child, no matter how many Genesis planes they create. (EDIT: And that's only if they spend all their time making Survival checks to support the little Resource Sponge!)

Point is, there's a few more variables involved.

@Quertus: I noticed you didn't consider it, that's why I reminded you, since it is a corollary to the argument from starting ages.

I'll grant it's a 3rd option beyond "Starting Age" and "Dead Babies".

As far as elegance goes, "everyone starts with racial HD only" and "everyone starts as a commoner" before finishing class training are both more elegant for populations at T=0 than "everyone starts as 1st level in some class". Reason being, with everyone starting from scratch, it is possible to use the starting age by class rules to determine the rate at which various classes appear in the world. "Everyone starts as 1st level in some class" requires additional assumptions to cover why those classes appear in quantities that they do, similar to DMG's demographic tables.

How to explain "elegance"?

Humaniods

Have two states: "Resource Sponge" and "Productive Member".
Resource Sponges follow these rules: ... and transition to Productive Members with 1 Class Level based on rules of Race, Age, AvailableClasses, Population, and Strategy.
Productive Members follow these rules: ...
At game Start, create X New Productive Member Humaniods with 1 class level as though they just transitioned from Productive Members
On TimePasses, check for Resource Sponge Humanoids with CurrentAge >= ProjectedStartingClassAge; transition them to Productive Members.
On TimePasses, check for Productive Member with CurrentAge >= MaxAge...

vs

Have three states: "Resource Sponge", "Racial HD", and "Productive Member".
Resource Sponges follow these rules: ... and transition to Productive Members with 1 Class Level based on rules of Race, Age, AvailableClasses, Population, and Strategy.
Racial HD follow these rules: ... and transition to Productive Members with 1 Class Level based on rules of Race, Age, AvailableClasses, Population, and Strategy.
Productive Members follow these rules: ...
At game Start, create X New Racial HD Humaniods with these rules for starting skills: ...
On TimePasses, check for Resource Sponge Humanoids with CurrentAge >= ProjectedStartingClassAge; transition them to Productive Members.
On TimePasses, check for Racial HD Humanoids with CurrentAge >= ProjectedStartingClassAge; transition them to Productive Members.
On TimePasses, check for Productive Member with CurrentAge >= MaxAge...
On TimePasses, check for Racial HD Humaniods with CurrentAge >= MaxAge...

2 data objects vs 3, 2 sets of rules vs 4, 2 TimePasses check vs 4 3, and a lot of redundancy in the code. OK, fine, there is 1 line of arguably inelegant "cheat code" necessary to make the 1st one work, but it's paralleled with a similar line in the other set.

I would never write the latter when the former was a possibility.

With complex multiple inheritance, you could remove the code redundancy, but then the object definitions would be rather inelegant.

Still, if you wanted no classes whatsoever until classes were unlocked, you could have those 3 states, but the Resource Sponge objects would always follow the same check of looking for available classes to determine whether to flow into Racial HD or a Productive Member, no extra inelegance introduced by starting the T=0 Humaniods off with Racial HD (except for the crossed-out line I accidentally added above).

So, all in all, just starting with class levels is the most elegant solution.

Related, the split you draw between "simulationist" and "gamist" is nonsense. In general, it is not a good idea to use Forge terms in context the Forge largely failed to cover. "Everyone levels by XP" is not more or less simulation-like, nor more or less game-like, than "population spreads are determined by tables". They aren't even mutually exclusive. We're instead talking of different ways to set up a simulation, specifically contrasting ways to set up the initial conditions for it. All methods, on a fundamental level, involve arbitrary authorial decisions regarding the initial populations, and "everyone levels by XP" is an additional rule on top that may or may not be in place in any version.

I mean, if you want your (AFB, Internet numbers) 10,000th Humaniod to spring from the womb as a 20th level character, because that's what the Population Table says you get at Humaniod #10,000, and you want to call that Simulating how things work rather than an oversimplification (ie, a Gamist abstraction), you do you. However the Forge may have done these terms dirty, I find them to have meaning, and I call the population tables Gamist, and tracking individual growth Simulationist (even if XP is, itself, arguably a Gamist abstraction).

In any case, being biologically long-lived or even immortal poses no real benefit for leveling or numbers of higher-level characters. The first reason for that is that every race that advances by class is capable of reaching high levels in their lifespan - the second reason is that the only real limit is the existence of sufficiently high CR encounters. Other classed characters are an obvious source of higher CR encounters. The third reason is that there will still be all normal reasons for hostilities: not everyone wants to watch you get to arbitrary levels, and a long lifespan on its own doesn't make a character more resistant to unnatural death. It's more likely to work the other way around: achieving noticeable level advantage allows reaching full natural lifespan.

Eh, no. Let's step through these.

The first reason for that is that every race that advances by class is capable of reaching high levels in their lifespan

Maybe.

Let's say the only method of leveling, period, is through random encounters, and the random encounter tables end at CR 1. Then NOBODY will be able to get beyond 9th level, regardless of their lifespan.

Let's say the only method of leveling, period, is through your wrestling scheme (I like calling your XP scheme "coed naked wrestling" and mine of killing animals in traps "baby seal clubbing", btw), or my "Hone your Craft" theory. In such a sysstem of earning XP, infinite levels are theoretically possible, as your (to use your idea) wrestling partner(s) hopefully win about half the time, and hopefully level up with you.

In practice... lots of annoying fiddly details, but imagine Adam and Eve, one (starting or) pulling ahead, therefore winning more often... but the other earns more XP for their wins... OK, it's probably fine, especially with a larger population.

Because of the way the XP tables are set up, rate of level gain while wrestling equal-level opponents is, all things being equal, static. But static at what rate? If it takes 1 year per level, vs if it takes 20 years per level? That's a huge difference wrt what level races can reach based on their lifespan.

And, at 1 year per level, a 1,000 year old elf is looking pretty scary if you ask me.

Let's say the only method of leveling, period, is through killing other "active participant" races. While this is arguably the most interesting "1 way" scenario, it's also the one least in keeping with the thread premise. But in this scenario, enslaving other races and farming them for XP is the dominant strategy, IMO.

Let's say the only method of leveling, period, is through the DMG demographic tables. Then "age" is irrelevant; it's reproductive stats (including how quick a reproduction rate the Productive Members can support with their Survival skill) is the most important set of stats in the game.

That last one is the only "only" that's likely to see play (and it really doesn't favor the Elves); otherwise, there's likely a combination of factors that can add to XP. But how those factors are calibrated (and little things, like whether Society is smart enough to use Maps) will determine how quickly levels can be gained.

The third reason is that there will still be all normal reasons for hostilities: not everyone wants to watch you get to arbitrary levels,

Anyone "watching" will likely be "allied" or "hostile" already (although I suppose the system / AI / whatever could allow more nuance than that). With a map size and exploration rate anything like this world, I imagine most races will be unaware of most races for thousands of years, minimum.

and a long lifespan on its own doesn't make a character more resistant to unnatural death. It's more likely to work the other way around: achieving noticeable level advantage allows reaching full natural lifespan.

I'd argue that Society, and creating Safe Zones (my idea of hunting all dangerous creatures until the Random Encounter Table makes sense - just "Humans" in a Human town, just Elves, Allies, and Animals in an Elven Woods, etc) do a lot to make a character more resistant to unnatural death. Also, if we're following RAW, outlawing "Businesses", as they invoke rolls on the "Natural Disasters" table.

But, yes, obtaining levels, which happens somewhere between "quickly" and "instantly" (depending upon the underlying system) upon reaching the appropriate age, definitely provides a nice buffer against many things that cause unnatural death.