Is throwing a cutter at age 12 harmful?

Nope, when thrown properly all a cutter is a grip alternation of a fastball and is thrown just like a fastball.

I was told that you release it like a football with the off set grip.

There are many variations of the cutter but generally you hold it with a offcenter grip and want to just think fastball as you throw it. The key is to be nice and out in front at release.

Key to all this is “When Thrown Properly”

Buwhite, the key to any pitch at all is “when thrown properly”. A lot of people have complained about problems with the slider, to name just one example, and I have had to point out to them that when that pitch is thrown properly it is actually easier on the arm and shoulder than just about any other pitch, and they really need to re-examine the way they have been throwing it. As for the cutter—I can think of no better example than the way Mariano Rivera throws it. As he does it, it’s like a variation of the two-seam fastball, but with practically no spin at all—it just “cuts”, and it gives batters no end of conniption fits. 8)