I guess in terms of whatever timeline logic/ratio/conversion you use/canon uses, sure, they shouldn't be older than that, but Emperor Hulkling had Teddy, Tommy and David drinking, and even mentioned them getting drunk. Which means they're all 21-ish, and I don't think editorial or whatever would've let underage drinking get through (and this doesn't even include Billy drinking with Loki in the Wotr one shot they got)

And with Tommy being able to drink, that would mean Billy's also 21. For Kate, I know her age is contentious bc of Gillen's "mistake", but let's say that she's also 21-ish. Eli's apparently law school age as well per Marvel 1000, so there's that.

I feel like the disparity in what should be and what's being written comes from just lack of consistent usage. I'm sure if they got used plenty enough, or at least to Kate's level, they'd probably be written as still teens.