I understand what you're saying about teenagers consenting to sex. However, it still doesn't make it okay for a supposedly grown up man or woman of 25 or 30 to have sex with a teenager. My problem with it is and will always be that a 15 year old consenting to sex with 30 year old is not the same as a couple of teenagers consenting to have sex. If it ever happens with my child, heads will roll. Bet on it.
Oh, at the very least a relationship between a 30yo and a 15yo is very, VERY squicky. There's often a power imbalance going on there, if nothing worse. It's just typically not nearly as damaging as actual child abuse. I've known some people who had that kind of relationship with an older person as an older teen or much younger adult. But that's how they tend to view it later on, I think: as a relationship they had at one point that was perhaps not healthy but that they did participate in willingly and from which they gained some benefit.