i work out frequently. since i dont have any weights or anything, i just whatever i can. i do push-ups, sit-ups, bicycle-cruches, some squats, pelvis-lifts and i work out my biceps by using my scool-bag instead of dumbells. my back and lower arms dont get much of a workout. my legs do, but they dont seem to be 'bulking' up, they just look skinny compared to the rest of my body (could some1 give me advice about those)

i'm a little bigger than i used to be due to the constant excercise and my mates are beginning to comment on it. most people complement me, but some say i'm obsessed. i'm gonna ask my PE teacher this as well, but:

Should i work out at this age? what are the advantages/disadvantages?