FBs from 6 and up..
Keep it straight with them though. "Just Sex."
BEFORE you fuck them..
Negotiating after sex is fucked up, and if she doesn't accept you've got a can of emotional mess in front of you..
So, Be straight up ahead of time, then if she declines, LJBF her.
If she accepts: put on Condom -> fuck.
It shouldn't be harder than that.
But FBs tend to decline after a few months.... and you always have the possibility she finds a BF, in which case you'll put her on ice, and if he leaves you thaw her again.
Just be clear from the start.
that's the only important credential.
Though if she's hot HB7,5 or more.. It's probably gonna suck a little if she gets a BF.... It did for me, but hey; then you go out and get 5 girls # and see if they are future FBs.
cheers!