1 1/2 to 2 1/2 is tough for me to age right now, tips....
Do they appear to be the buck's first or second set of antlers? Fawns born six months prior will not have antlers, and 1-1/2 year bucks will have shorter spikes, generally speaking.
IMO, much of it comes from simply comparing the characteristics of the deer you see. You'll begin to notice the differences in appearance and pick out the ones that lag their peers.
It also helps to spend time in the woods during the off season, perhaps while hog hunting, and just study the deer you see.