Mug era. Older players can keep playing until they're over 30 because competition is a freaking joke.
Moya is the perfect example.
Yup, Moya is the perfect example. That such a weak player could become number 1 tells you all you need know about the late 90s. Alternatively, in the early 90s a 39 year old made the US Open semis.
Anyway, looking at the graph it seems to be something that's happening for 20 years now. Most players now peak after 25, with the exception of the very best.