A friend of mine who was well into his 60s at the time told me the brain turns to mush after you hit age 60. A month later, he won the State Championship, beating three players each less than half his age, and two of whom were each 200+ points clear of his rating at the time. I hadn't beaten him yet at the time (I have since, but he's still the stronger player), but even at that early stage I could see our games getting sharper, deeper, and generally more entertaining. I could tell he was trying to bring up my level of play to the point where I was a real test for him - and that told me he wasn't really ready to give up on himself.
The only thing that holds you back from further improvement is complacency. The game of chess is a lot more fun when you're working towards improvement - whether it's your own or your opponent's. There's no reason to lament the passage of time - enjoy the time you've got, life is a lot more fun when you do!
Fat Freddy's cat died the other day I was so sad I phoned fat Freddy he was traumatised