The CNT players do not need to maintain the body that long because the pressure of the incoming youth is too high and before they need to think how they will manage their body for advanced years in their career, they are already outplayed so it does not matter.

As said earlier in the thread, it's all about the players reservoirs for each national team: if they are all the same size relatively to their ITTF share (a national team is a national team),  China's is so fuller that the ensuing pressure pops new dragons that kick the ones in place out.

One additional note: when an Euro player loses a match, he does not get pointed at and questioned about their participation to any international competition; but because their players reserves, a loss has much more significance in China and carries heftier penalties; the pressure is so high on them and that's why they can have those godly moments in their short careers.

The OP question has to be temporized though: it is less and less true that Chinese players won't play late. Ma Long might win the Olympics and WTTC singles 2021 even if it looks less and less likely. I can't imagine FZD any weak by the time he is 30, especially when like Ma Long, his tactics evolve towards a more mature and complete game, whatever that means. Just like Federer, Nadal and Djokovic, Ma Long and FZD might take advantage of those new body maintenance techniques, outlast the typical career span of a Chines CNT leader and start a new trend where CNT has leaders in their early 30s on a regular basis. After all, little geniuses like WCQ do not pop that often.