Very hard to tell because you can only see the front of his body. My guess would be at least 5.5 but very hard to tell without being able to see his whole body
Nice but you still can't determine age from that picture. Is he mature? Yes but beyond that you could age him at any number you want to because it would be a guess.
I agree that is it hard to tell from that picture alone. He is not a yearling though, that's for sure