"Top-End" 2 1/2's typically appear much as a 3 1/2, and one reason is their antlers (which seems to be the primary focus of many hunters) will in fact outscore the average 3 1/2.
I suspect a majority of above average 2 1/2's are "early born", by which even born a month earlier than average can allow them to express much more of their genetic antler potential at an earlier age.
Most of any particular buck's genetic antler potential is not realized until he is at least 4 1/2. But if born a month or two earlier than average, if living in a low-stress environment, and if having high-quality food resources, these bucks can express more of their potential at a younger age.
IMO, assuming you consider larger antlers a better trophy, these are the very best young bucks to allow to live another year or two. These particular 2 1/2's may express about as much of their "potential" at 3 1/2 than many would by 4 1/2.
While antlers should not be the primary focus in trying to assess a buck's age, a particularly good antler clue in sorting 2 1/2's for 3 1/2's is "mass". It's certainly not fool-proof, just more consistent than most other antler characteristics between 2 1/2's & 3 1/2's.
If you're somewhat undecided between whether a buck is 2 1/2 or 3 1/2, should the antlers look "skinny" relative to their size, that feature is more characteristic of a 2 1/2 than a 3 1/2 (particularly regarding the antler circumferences about an inch about the hairline). A good field "gauge" is comparing the buck's eyeball diameter to his antler diameter.
Also, "top-end" 3 1/2's also commonly appear to be 4 1/2's, until they're killed and their teeth say 3 1/2.