Having had a bit of a go during the Hainan roadshow last year I firmly believe the current medical "standards" they have make it difficult for many and that age - even if above average in fitness - only makes what at times appears to be arbitrary and variable standards even more difficult to achieve.
Whilst this is probably the same all around the world I'd suggest it has a greater negative impact in China than most other countries.
Then of course you have to prove yourself again every 6 months, talk seems to suggest some strange and totally unexpected things can be "discovered" - your job security is only as good as your next medical.
Cheers.