A relatively simple system for starting ages I saw someone suggest recently was basically to have the player select at character creation whether the PC is Youthful, Mature, or Aging. Note that this was for a simpler version of D&D without a full skill system. I think it might have also been in a context of a rule where every time you fail a death save, your death save value permanently goes up by 1.
Youthful characters get a base death save of 8+ and +10% to earned experience.
Mature characters get a base death save of 10+ and to choose a professional career they had before adventuring. They get one pricy/valuable item or set of gear related to that profession (could be a horse, a small smithy or workshop, or the like) and are assumed competent and knowledgeable in relation to their profession. So they can use those skills and knowledges during play.
Aging characters get a base death save of 12+, the professional career stuff, and a useful NPC relative or servant. The equivalent of a henchman or sidekick character.
As for aging during the game, I'm a bit leery of setting any arbitrary break points. I think what Pendragon does is pretty good. IIRC every year after the character turns 35 you make a check* to see if any of your ability scores degrade. It's a 2d6 table, IIRC. More extreme rolls result in more stats going down. Once you know how many are degrading, randomize which ones and they each decrease by 1. Of course, you can also increase ability scores by training in the winter phase in that system, so you can offset decrepitude, albeit at the cost of not putting those advancement points elsewhere.