Can cracking youe back in your early thirties be harmful?

Depends who does it: If this is being performed as part of a needed treatment by a qualified professional (do, md with extra training, dc, pt, etc) then it should be fine.

Unlikely: Cracking across the back is not necessary harmful. It of course depends on what is cracking. Slippage of joints and soft tissue more often occurs in flexible people and some practitioners will manipulate and crack the back. As long as the cracking is not due to structural damage, but to movement of normal components it is unlikely to be harmful.