It's against the Law and if you were to be caught then you would be charged with statutory rape and you would be placed on the Sex-Offenders list. That would be the main problem; Sexually Transmitted Infections are also a problem and then there is pregnancy. "The Pill" and condoms cannot...
I'm going to say 5.5 m/s... but I can't be sure.
Rearranging the equation makes me think it's force times mass but I think it's Force/Mass...
A=F/M being the forumla I use.