Regarding A/2+7, I should only go as young as 17. This is fair, as far as open dating goes. I'm perfectly comfortable dating a mature 17 year old, and as far as upper range goes I'm fine with girls into their mid-to-late 20s.
However, I would, if the law was not an issue and no one would know, have physical relations with a girl as young as a REALLY hot/physically-developed 13 year old, realistically as young as 14. But that's a dream compared to reality, where I wouldn't go any younger than 16. I would barely mess around with a 16-year old at that, even though it'd be perfectly legal in my state.
*Edit* Of course, everyone is different. I met this really awesome girl that seemed very smart, funny, etc etc. She knew my age but when I asked hers she (slyly) avoided the subject in a damn-impressive fashion, in hindsight. We kissed or whatever, and was holding hands the whole time (at this big bonfire), and it wasn't until the next day that I found out she was barely 15 years old, when I was right about to turn 20. Goddamn my luck. I'd still go for her if we didn't both agree to put an end to it for social reasons.