Sleepovers

At what age did everyone start staying over at their significant other’s house or vice versa? And if you had to get permission from your parents how did you go about it?

My boyfriend and I are both 20, and I know if I were to ask my mum if he could stay over she would say no, because she would presume it would lead to sex or she would just say “why does he need to sleepover, what for?”

She’s let us go on holidays together for four days to another state. So I don’t understand why that’s ok