Spork, I was woke up this morning, and happaned to catch the end of a dicussion on 3AW... A pyshologist was a guest star, and someone phoned in to talk about their 22 year old son having a sexual relationship with a 16 year old girl. I thought it was weird I was listening to it the morning after I read this thread, maybe it was you (considering geographical location), but then how many people are in Melbourne?
Personally, I think this shows immaturity on the parents behalf... phoning a radio station and publically broadcasting their son's personal life, and only looking at it from the Christian perspective. To be frank, there's worse things in this world than sex, but that's irrelevant.
It's right if you, and her feel right about it. Simple as that. It's not right if other people are okay with it, and on the same token, who cares if people look down at it. It's not their life! You've got to work it out if you have a problem with it, however. You're asking the question, so obviously you are thinking about it. Do you think it's wrong? Maybe you should sit down and talk to her about it... the people that know the situation best is herself and you.
There really isn't a set age difference that is alright. It depends largely on the people involved... you'll work it out. I don't believe it's morally wrong, however, so don't beat yourself down thinking that way.