The legal age - so 16-18.
While your brain does tell you its weird to see an 18 year old with a 35 year old, that is just because you yourself see 18 year olds as children and 35 year olds as adults. In reality, people have massively different maturity levels. I hooked up with my future wife when i was 15 and she was 18. I was the mature one in the relationship and was, in her words "her rock." - we've been together over a decade now and are married.
But at the time, people responded with either disgust or the classic south park 'nice..'
For me, at 15 years old, the idea of hooking up with another 15 year old was fucking disgusting. I've never been someone that just wants casual sex. I'd refuse to fuck Scarlet Johansson if she cornered me and asked me for it. Thats just not me - i need to know the person and holy fuck i do not want to know a 15 year old girl.