Nyu...Ash started out at 10. I remember in one episode Misty said it had been a year since they met, making him at least 11. That was before Orange Islands, though, so at least two years have to have gone by since then.
Ash ~ 13-14
Misty is older than Ash, because she said so at one point in time...so she's got to be at least one year older, putting her around the 14-15 range.
Since all trainers start their journey at 10, then Gary is the same age as Ash. ~13-14.
May is 10 since she just started her journey.
Any other character ages would be random guesses based on looks.