I think that what counts as sexual abuse changes as society changes... Ian Hacking's 'Multiple Personality and the Sciences of Memory' is an interesting read on that score.

He considers how nowdays many things are considered abusive that never were before. Oftentimes calling something 'abuse' is a call for social change... All the way back in the history of our ancestors when the average lifespan was around 45 it was not uncommon for girls to be married off at puberty (around 14). It didn't count as abusive in the culture... But maybe we can call it abuse because we are more enlightened? Who knows...

I think that situations vary... I know a few 30 year olds with the emotional intelligence of teenagers and a few teenagers with around the same emotional IQ. I think it could indeed be possible for a truely consentual relationship to develop... But then I guess in the majority of cases in our culture such an age discrepancy signifies something... But then while an age discrepancy of say a 60 year old millionare and a late twenties trophy wife certainly also signifies something we don't consider it to be abuse...

Regarding the underage people acting of age...

Maybe there is a cautionary tale in there about casual sex...