I don't think Regina is that much older than Snow. Snow was supposed to be around 12-13, when Regina became her adopted mother, and at that point I think Regina was supposed to be around 17-18. It gets tricky because they've got the actress playing the same character at all different ages, but I definitely think that when Regina married the king she was meant to be no older than 20. When the curse took hold, Snow was 23-24, and Regina is supposed to be in her early 30's? The actual dates are tricky, but I don't think they're intended to be any more than 6-7 years apart. Which is part of why Regina being Snow's "mother" is so creepy.