Using the formula (half my age + 7) I come out with 17 and a half. That's about half a year too young, going by the law, but then the equation breaks down around the age of 18. It works from the age of 14 to the age of 17, breaks down from 18-21, and then works pretty much from 22 on. That said, at 21, I'm pretty hard pressed to find an 18 year old that I can see as serious relationship material; there is so much growing up that happens between 18 and 20, to the point that 18 to 20 is a much bigger maturity gap than 16 to 18. I tend to find women who are 19 or older a lot more interesting than 18 year olds, although as always when dealing with maturity, different individuals are different.