Here's a link to some aging tips: Galvayne's Groove In Horses: Photos
The Galvayne's Groove isn't entirely accurate, but it's pretty close. There are also other 'clues' to look for [including angle of the teeth, etc] but, I think, for a horse between 15 and 30, that groove is possibly going to be the most easy to spot.
If you can get some pictures of his teeth, there are people on this board who are pretty adept at guessing ages based on teeth pictures. :)
However, nothing about teeth is "fool proof". For instance, my 28 [29 in Feb] mare has absolutely no groove anymore and her teeth, angle-wise, are still quite upright. However, they are adult teeth, albeit short - due to age, which, combined with the fact that she used to have a groove, all would confirm her age if I didn't know. But, to someone who wasn't experienced with teeth, she might appear to be younger...or older, than she really is!