So okay, I will say straight up: I have not read Champions, Waid is probably out of touch, I can't really discuss the finer points here, so this response really just is my gut check/general thought on the bit I bolded in particular, but imo if the writer isn't nailing their voices, I wouldn't assume getting someone younger would solve that problem. The whole point of being a writer is inhabiting other people's shoes, I don't think you need to be a specific age to have that skill. I do think you either have some voices or you don't, and some people just write young people poorly. I'd have a hard time saying specifically that a younger writer would be an immediately better option though. I mean Joe Hill is 44 right? L&K started coming out in 2008 I believe. Dude was already in his mid 30s, and finished in his 40s. The book has a lot going on in it, but it's effectively a coming of age story for the Locke children, who rather from Teenagers, to younger, and it's fantastic.
So anyway, I totally get that a writer can become out of touch, but just a thought about the idea that the book would be inherently better from a younger writer. Age might inform the tools the writer has at his/her disposal both ways, and I think you could be just as likely to find an under 35 who also fails the task, as an over 35 who would succeed (and vice versa etc etc). Not really disagreeing with you at all then, but more just a jump off point for a further thought