By law it would be considered experimentation, as long as it is consensual.
It is, however, really, *really* vile. And quite immoral on the part of the fourteen-year-old. I mean, chances are, the twelve-year-old hasn't even hit puberty yet.
The most sexual thing I did at 12 was pretend that my Barbie's were having sex. A bit weird, I know, but not as weird as a 12 and 14 year old gettin' it on!
If a 12 year old is having sex its just not right. At that point whether its illegal or not is secondary to a morality system. A 14 and 12 year old are WAY too young to be having sex.