It depends on how old you are - if you are 25 and like a girl that's 22, or you are 21 and like a girl that's 18, etc... that's fine. If you are 18 and like a girl that is 15 - then that is not ok, because you are legally an adult and she is still just a child, and by having a relationship you are risking possible felony charges and/or jail time.
I am almost 8 years older than my wife. But as said before, it depends on your age and hers. Not the difference in age. Just her age. We married when she was 17, and courted for a year before. It was all very proper. I wouldn't say you should "date" till after your both adults. But if you care for each other, do the right thing. Get to know each other in the presence of your parents and hers.
It does depend on how old you are but it also depends on what kind of a relationship you are talking about too. Are you talking about having this girl as a girlfriend? I'm just trying to answer the question in a different perspective. It has been said that a woman on the average is more mature than a man her own age by 4 years. So if you are both consenting adults, a man having a relationship with a woman 3 years younger is actually a good match.
Although this may depend on several factors, it is not actually bad considering the age difference is moderate. However, it is important to ensure she is not considered a minor by law. Generally, society sees it as normal for men to date younger ladies, but looks at the reverse as suspicious.