My credit score is just under 750 right now and I haven't had any significant bills for a while. It's all because I co-signed with a loan my dad got that has been counting toward it.
I still can't open a line of credit above $350 tho. That score is so bullshit.
It's because you don't have any credit history (the co-signed loan doesn't fully count). Go get a credit card and pay off the full statement balance every month. That will build credit history and account age. DO NOT carry a balance, that incurs interest. Simply pay off the statement balance every month when the bill comes.