14 and 18? Major sketch factor. 18 is a legal adult. 14 is barely past puberty.

Yes, girls mature faster than boys, and yes, four years isn't a huge difference...down the line. But at this point in life, four years counts for a lot. You're at different points in your life, different maturity levels. It's a bad idea all around, not to mention very illegal.

>>2 has the formula right; half of your age + 7. It does make a difference.