I recently read an article citing a Mississippi study on this topic. Basically, they found their bucks peaked at 5-1/2 years old. Bucks scored 63% of their eventual peak at 2-1/2, 77% at 3-1/2, and 92% at 4-1/2. Of course these are averages in a penned group of bucks. In the wild, you're going to have more variables.

According to those numbers, a 2-1/2 year old buck would improve his antler score by 20-25% at 3-1/2.