Not the best at aging, but...... I am going with 3.5.
Even though he is a northern deer, his neck is to big to be 2.5 and to small to be 4.5(its very close to the brisket, but not quite), his legs aren't overly long but they aren't short in appearance like a mature deer, he is missing the swag in his back and pot belly of a 5.5+ deer, but he has a small hump for his shoulders even with his neck/head up. I get tired of hearing this, but I think it rings true with this deer.... give him another year and he will be great. Give him two and he will be outstanding.