I can't help feeling he is a little too old to suddenly be a very good player.
You have a very good reason to feel that way.
If he keeps playing the way he played last year, he'll be an exception of the rule more than a rule itself. Kinda like Mauro Camoranesi who "suddenly" became a very good player at 26, when Juve bought him from the relegated Verona.
But for every Camoranesi there are 50 Almiron's, so as i said, you have a good reason for your fear.
I'm hopeful about D'Agostino because the talent was always there. He, De Rossi and Gilardino were considered the most talented youngsters, after Cassano, of their generation. But while the other three did prove their talent to the world, D'Agostino needed 5 more years after he stopped being an u-21 to do something good and something everyone expected to see from the extremely talented youngster.