After browsing a few hundred pictures, I've come to the conclusion there's two ways to portray air elementals: 'mist person' and 'tornado person'. Ironically, both these things are closer to water than air.
All air elementals have air mastery, which gives airborne creatures a whopping -1 on attacks and damage. They can also turn into whirlwinds, which is actually pretty nice as long as you weren't planning on making AoO's.
Small Air Elemental
The smallest size these critters come in, at 2 RHD. Elemental RHD alone are pretty bad, especially air RHD, but they do give some useful immunities. Ability-wise, smalls trade 6 points of intelligence for an equivalent dexterity bonus.
Small elementals enjoy (perfect) fly speed, as well as some natural armor (the logistics of which I won't dwell on). The 1d4 slam is better not used, but the two bonus feats are very welcome on the kind of build that would want to be this monster in the first place.
Flight is a major ability at low levels, especially at this maneuverability. The whirlwind is nearly useless, but the two free feats are pretty huge. +1 LA for now, though I do admit it's close to +0.
Medium Air Elemental
The next step up is 4 HD.
Mediums have larger physical bonuses and a size increase: apart from that they're very similar to small elementals. I guess one could make a case for +1 LA, but I'm assigning them +0.
Large Air Elemental
8 HD this time. The DR is pretty neat, but the increase to ability scores is much smaller compared to the small --> medium jump. The extra slam isn't that impressive either. I doubt there will be many medium creatures to whirlwind around by this level, too. -0 LA.
Huge Air Elemental
The 8 HD increase should be enough to write this one off: if it isn't just remember this thing's ECL is more than twice its CR. -0 LA.
Greater Air Elemental
Extra DR? I'm sure this'll help me fight solars, titans and great wyrms. Did I mention all three can already summon multiple GAE's? -0 LA.
Elder Air Elemental
If I'm not mistaken, the 'update' here is a net +6 to stats for three extra HD. -0 LA.