This assumes the priest didn't report Romeo to the polizia as a child molester, thus derailing the entire plot...
Romeo would probably be fine. He wasn't that much older than Juliet. Shakespeare on the other hand... The source of the play was The Tragical History of Romeus and Juliet by Arthur Brooke. In that, Juliet was 16. Shakespeare cut her age by three to make her 13 for...."reasons"
I don't really see a reason why shifting Juliet's age back a few years would have been simply a result of the standards of the time. Aging older women back to be younger would be something that you would see more often than aging an already young girl back to an even younger girl if that were the case. There are plenty of examples of varying ages of female characters in Shakespearean plays. Including much older. It's generally accepted that they were all played by younger boys and there's no real evidence that I can find of him changing the script due to this particular quirk in acting. In fact, Shakespeare was generally regarded as quite brilliant for his ability to build the illusion of these formidable and complex female characters that were actually played by boys.
Good point, that's just the reason I've always heard given for that particular change, but when you put it like that it doesn't make much sense... Maybe he was trying for an Aesop about marrying too young? EDIT: (I'm fully aware I'm grasping at straws here...)