Woah, 16 year olds... reaally????
I think the calculation is half your age plus 7 is the youngest you should go.
Im 25 so thats 12.5+7 = 19.5
sounds about right, but seriously any guy over 23 with looks on the scale of 6 out of 10 or higher could bag a 16-19 year old so easily... By far the hardest girls to get are the ones of your own age group. I've shagged a few young girls and it is not fulfilling. Just makes you feel depressed for using and taking advantage of them.
As for old chicks... depends on maintenance, or if there is some other hot factor like shes your ex's mum.