I think 2486 makes a lot more sense, too. Fillion was about 30 while the show was airing, so I always imagined Mal was around the same age.
I can also buy Simon being 27. He's a full-fledged doctor, not just a med student, and seems to already have some experience working in hospitals. He's just one of those people who looks young. Sean Maher was also 27 at that time, if that makes any difference.