DM could use some advice

[Serpentine]

Don't run a game you don't want to. Put your foot down on it: "In my game, I'm willing to allow dragon characters of this age and younger, and/or of these CRs and lower. If you want to play a game full of adult dragon PCs, you can DM it, because it's just not the sort of game I want to run." Alternatively, you could just put it in the basic game information: "I'm running a game of X level. You can be whatever you want, but your ECL must equal that."
I would hope that, as fellow DMs, they would understand you not wanting to run a game you don't want to run.