Hmm, my first boyfriend when I was 16 was 22, I was a very mature 16 year-old, there was absolutely no power imbalance, we had a lovely relationship for three years which I ended when I left to go to University. I'm still friends with him 35 years later.
It entirely depends upon the individuals concerned. I wasn't a child, or in any way vulnerable when I entered the relationship, and there was nothing at all strange about it.