IM Silman's latest article at Chess.com is "Old Age, Great Chess!"
In it, he takes as the centerpiece of his discussion IM Anthony Saidy, who is something of a legend in US chess, with his career stretching from the Fischer era to the present. While it's intended to be an homage to Saidy, there are also some points made about age and its affect on chessplaying ability. For those such as myself looking to improve while past the teenage years, there are a number of interesting observations in the comments section that go beyond the article's focus.
Basically, there's no need to give up hope. Even if technically speaking we may have passed our absolute peak potential in terms of mental energy and focus, with sufficient time to devote to effortful study there's still an upswing possible on the learning and performance curve.