Probably about as old as you think they are and probably not as old as they claim to be.
Thanks to one very sophisticated Google search (described here) by search engine hacker stryde.haxwe can see that He Kexin was probably born in 1994 rather than in 1992 as reported making her 14 years old instead of 16 and therefore not eligible to compete in the Olympics.
What news! We wonder if that hacker could manage to figure out what drugs Michael Phelps is on. Now that would be a discovery.