How old should she be?

To have a serious relationship? I know my children have had boyfriends before, like my 12 year old had a boyfriend, and my 9 year olds and my 6 year old say they have boyfriends. But the other night I walked into my kitchen and my 14 year old daughter Hannah who's in 8th grade was sitting ont he kitchen counter making out with her boyfriend, and I mean really making out. I mean I know she's kissed boys before, but he looked like he was ready to take her top off. I know Hannah would never make a bad decision like that, but I guess because even though she's my oldest, she'll always be my little baby and with Hannah it's my first time gong through all this so it's new to me. So I guess what I'm asking is how old do you think a girl should be to be having a serious relationship with a boy.