Personally, I wouldn't mind the age gap assuming that the two characters are close to equal in terms of mentality and maturity. If he's got his life put together and she's just a ditzy teenager then it might seem off to some people. If you really want to you can change his age to 23 like others have suggested, but I'm not sure that you really need to.
Also, I'm not sure why everyone is saying to lower your MC's age to 18. I think 19 is an acceptable age for your genre. If your target audience is the upper age range of young adult, then it's actually perfect. She would be out of high school by that age hopefully (I was), maybe even starting college, but she would still be a teenager. That way the younger crowd will still be drawn to her, but so will the older crowd. She's still young, but she's got enough adult in her at this point to stand strong.