Difference in developement?

At 13 1/2 years old, normal boys will vary considerably in their stage of development in puberty. If all you've noticed is a little pubic hair, then you are right.... other boys of your age will be further developed than you. However, you are still perfectly normal and that hair growth means that you have definitely started puberty.

The most spectacular changes for a boy (getting rapidly taller, underarm hair, a deepening voice and the ability to ejaculate) come late in puberty so many boys, as they start to develop, worry that there is something wrong because not much seems to be happening.They're perfectly normal though and just have to wait for, perhaps, a year or so before major changes occur.

The good news is that, if you're already 5' 8" and only in early puberty then you still have a lot of growth left in you and, depending (probably) on how tall your parents are, you are likely to grow quite a bit taller. You're quite likely to make 6' or more.

You're probably a bit early in puberty to start working out, David. It won't make much difference now because your hormone production is still at a low level. When your development is well underway ( that is, you have fairly well developed genitals and, probably, quite a bit of pubic hair) start working out and, if you keep at it, you'll soon bulk up. Hope this helps!