Fran's age currently in the series has yet to be revealed. I've heard many different answers on this. Some say that (in the future) he's some where in his mid-twenties like Tsuna and his friends would have been. Though in my opinion, in the present time he seems younger than Tsuna and his friends, but not by much. So he should be about their age. I hope that helps. But it has yet to be released.