Do we not assume that PCs are not average dumpy joe? I mean, yes, there are plenty of middle aged people who are talentless/not the top tiers of their professions, but frankly, most 18-25 year olds aren't at the pinnacle of their careers either. I admit, I'm not edgy enough to want to play average joe PCs, so I guess I kind of assumed that PCs were going to be exceptional regardless of age.
Also, can I giggle a bit at 40 being considered old and grizzled? 4 months before I turned 40, I gave birth to child #4, and had run 5k every other day through the first trimester of pregnancy, and worked as a massage therapist (on my feet, doing deep tissue bodywork) until 2 weeks before I had my C-section. Hell, all the marathon runners and iron man qualifiers I personally know didn't even start that shit until their late thirties (though again, these people I know ARE pretty badass and would probably qualify as PCs.
But if you're not going to "stupid, inexperienced kid" handicap PCs, I really think you shouldn't turn mid-life PCs into elderly people. But I do agree that I am kind of meh at the idea of huge reward to older PCs UNLESS you have a compelling story reason to want a lot of older people in the game (which honestly I can't think of many, but I'm sure someone more creative can). People RPing like their 35 year old PC is elderly AND people who want a Doogie PC with 2 PhDs and who is a star athlete at 18 kind of annoy me about the same amount.
And that still doesn't solve the differential problem either, it just gives some people a little bit of a head start to reach their max potential.
I think that you have to also look at the lifespan you expect from the game as well. If you have slow but steady XP gain, you could tailor it to the idea that the game has a finite lifespan (I'd put it at 2-4 years). You could have a policy of PCs riding off into the sunset (or going for a long walk during a blizzard once they reach a certain age/capacity) at a certain power level, but if you pace it right it's exceedingly unlikely that anyone would have to do that before the game comes to its end. Maybe honestly HAVING an end in mind (though that will cull some of your playerbase since a lot of people have trouble with the idea that they have a fininte length of time to play their pcs, even if they KNOW that's the case, it's worse if they go in with people acknowledging it. I don't share that dislike, but I accept that there are plenty of people who do, so it's a factor in considering what audience you want.).
Or if you want to run a high powered game, I think it's awesome to just embrace it. I've enjoyed seeing some creative CGs were people were allowed to borrow their entire lifetime maximum XP in cg if they wanted to (though usually the respects were severely limited, like 1 time ever, ir that). So people COULD make a god right away if that's the level they wanted to play, or if they wanted to RP out talents unfolding and the like they could too. I would imagine you'd have to have storytellers willing to take on lots of different levels though. Or make the game a sandbox so that burden isn't on staff.
I think there are stories that can be enjoyable and fun at any level. It's just a matter of what stories have the most support on the game (and how that evolves over time). I am not sure that everyone has to be overly concerned with catching people up necessarily (unless it's important or desirable for runners to get new blood in). And I think eliminating XP rollover between PCs probably would do a lot to ensure that there was either a stable population of pcs at the same level OR there were many people at the lower end of power/level as well. I don't always think the attachment to previously earned XP on an old PC or allowing people to earn XP on a series of throwaway PCs that they the reap it from to pour into another PC is very healthy for a game a lot of the time. But I understand why people would be reluctant to not have it, since that can be pretty standard in some subcommunities, and it's often one of the few ways that any player can feel some continuity of "ownership" as a player on the game.