Meaning that they get worse rather than better with age. That’s the conclusion of a long-term longitudinal study of peer reviewers in medicine, discussed over at The Scholarly Kitchen.
I’m not inclined to compare the results to my own personal experience, simply because I doubt anybody’s personal experience includes enough data to meaningfully estimate what is a relatively small effect. But FWIW, the results do seem plausible to me. They likely reflect some combination of senior people being less in touch with the current literature, and less inclined to put as much time and effort into their reviews as they did when they were younger. (Note that I didn’t say “Senior people have less time for peer reviews than they did when they were younger.” Your time allocation is always your choice, as illustrated by the fact that people of the same level of seniority within the same department often allocate their time in vastly different ways.)