But you’re not improving your credit with a debit card because you’re not borrowing money with a debit card, you’re simply using your own. If you’re not spending money you don’t have, i.e. borrowing, then you’re not building credit.
Not everyone maxes their credit cards out when they first get them. I, too, got my first card when I finished college and haven’t had any problems. My credit score is a 720 – and I’m being penalized for a short history! I absolutely regret not getting one sooner.
If you can’t control your expenses, then no you’re not ready for a credit card, but the OP didn’t give any hint that this is the case. For a responsible person, you should get a credit card ASAP, and just use it wisely.
Seriously, if I ever have kids, I’m getting them a credit card as soon as I leave the hospital, and shelving it for 20 years so that when they want to buy real estate at my age, they have an excellent credit score and can secure a decent interest rate.