When does age matter?

It's not so much the age difference in years that will be a problem - it's the fact that she is only 18. If she was 38 and he was 59 I wouldn't see a problem if they were both happy. There is little difference between a 38 year old and a 59 year old but there is a huge difference between an 18 year old and a 39 year old in terms of life experience, etc. It cannot possibly be a balanced and equal relationship.

That big would bother me. I think if they are old enough to be your parent that's too much, but I don't have a problem with age difference 12 or less. If it doesn't bother them then hey I wish them all the happiness in the world. I just hope it's not to bumpy of a road.