Personally, I'm not fond of penalties, and I'm convinced stat modification isn't the best way to represent aging. In the way I mean to approach this, youth gets you something, and old age gets you a different something. There's no penalty except perhaps not having that something that youth had now that you're older.I'm not entirely convinced that stat modifications are really the best way to represent aging. I wonder if it might make more sense for older character to do something mechanically similar to choosing additional backgrounds to represent their additional experience. Or, perhaps, choosing skills to gain expertise in. That's really what has happened. You've had time to burn through a lot of downtime activities.
I'm not sure how best to represent the effects on the body of aging, or from just being out of shape and out of practice. Flat penalties are simple, but they don't seem like the most accurate solution. I would almost go for something like atrophied class abilities. In fantasy stories, we often see things like old fighters who are still very deadly, but they can't do everything they used to. Or old wizards who can cast many spells, but no longer have a grasp on their most potent spells. Maybe you keep your proficiency bonus and your hit dice, but everything else starts to fade after decades of disuse. You're still a 13th level Fighter, but you've only got the class abilities through level 5.
At the venerable stage where you're so old that you need dedicated care, I kind of feel like that would best be represented with steadily progressing permanent exhaustion.
Yeah, I really don't want the 3e stat adjustment rules. I would rather take your proposition upside down; old age doesn't get exhausted easier; youth is harder to exhaust.Anyways, to the OP:
If I make an 80 year old 1st level character, the stats I choose kind of represent that. I used to use the 3.5 ageing rules. They were neat but they lead to some min/maxing (I want to play a 70 year old wizard to get higher INT!)
It would be interesting to, instead of stat penalties, you made exhaustion trigger more easily. Maybe you have to do a forced march after 7 hours instead of 8. Maybe you get disadvantage on con saves when doing strenuous, long term activities.
In which case you've run aground before you've left the dock.Personally, I'm not fond of penalties,

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.