Oh I don't think that he'll have much of a problem. Persona is more about personality than looks... I know plenty of 'older' people who are still young at heart. Some of them look young and some of them look old... but the persona they have shines through whatever age their body is. I think James is one of those people.
Of course there will be always be people who focus totally on appearance and ignore everything else. For those people that can't get past a few lines in his face then I guess his appeal will probably go down.
But if there's one thing about James Franco that I can say for sure (without actually knowing him personally) it's this: He's proven himself to be a chameleon. So if his age 'ruins' his boyish ability, I am pretty sure he'll find a way to adapt and keep people entertained and interested in him.