It looks like it might be a little long, especially if you force it all the way in. The prostate is usually about 1-1/2 to 2 inches inside, but the best way to figure out is to see for yourself. Get yourself excited and insert a finger into your anus. Feel the "front" side of your rectum until you locate a bump, and that's your prostate. Note that some massagers are designed to go around the prostate and stimulate the top as well, but for that you want a pretty firm material (stiffish silicone is good).
BTW "around the ages of 10-13" isn't really old enough to be here. Sorry to rain on your parade, but that's the law in many areas. Hopefully you're older and the age is just for size comparison.