I was very close when I was 17 (only a few days before my 18th bday.)
I was dating a girl who was 16 and absolutely gorgeous. Last time I hear anything about her she was modeling (primarily underwear.) Anyways her mom had her when she was 15. (That makes her 31 for you non-math majors.) One day I went over to my g/f's house to see if she wanted to catch a movie or go out. Turns out, that she was over at her friend's house, but the mom invites me in and tells me to have a seat before she tells me this. (Her mom was always really cool with me, and my g/f used to always get mad because she thought her mom was flirting with me. When we'd argue she'd actually, "Why don't you just go fuck my mom.") So after the g/f's mom tells me this she sits down next to me on the couch and starts talking to me. Her mom is crazy hot and cool, so I make with the conversation. One thing leads to another, and next thing I know I'm making out with my g/f's mom. THis is absolutely great for a 17/18 y.o. as her mom is VERY nicely developed. So my hands are everywhere (very happy memories) and too my surprise hers are in just one place - down the shorts and boxers. After a few minutes of this - I was crazy excited and rock hard - I started to have second thoughts because I really liked my g/f - who I already thought was great in bed, so I didn't want to fuck things up. Anyways I stopped, the mom had this "Are you sure?" followed by, "Oh shit, did I cross a line?" face and I eventually left. Luckily we didn't really talk about it (I thought about it ... Hell it's been fuel for private sessions for years!!!), but we never really talked/flirted as much afterwards. I don't think the g/f ever had a clue, but I'm sure she noticed the difference in the relationship between her mother and I.
Anyways, broke up with the g/f a couple of months later and for the last 8 years I have wished that I had gone ahead and hooked up with her mom. You can't beat the "Family Bonus Plan!!!"