My bet is that to the degree that neuroplasticity is involved at all, it's effects are largely dominated by more mundane effects, such as the fact that as you get older you have kids and other commitments that keep you from spending as much time learning new things as you used to.
Likewise an 80 year old that sees themselves as having less than 20 years left might require a higher bar for the payoff for dedicating themselves to learning something new than a 20 year old that sees themselves as having 80+ years left.
> 20 year old that sees themselves as having 80+ years left
Any such 20-year-old is delusional; becoming a centenarian (let alone having good enough health to appreciate it) is not much more common than winning the lottery, at best.
Likewise an 80 year old that sees themselves as having less than 20 years left might require a higher bar for the payoff for dedicating themselves to learning something new than a 20 year old that sees themselves as having 80+ years left.