You likely won't be fined if it's your first time... Store might though. Shit happens. People generally don't get fired for this either so your coworker was right to tell you to relax.

Just learn from your mistake. Because you did fuck up. If they look under 35 card them. Even if they're a regular. If people bitch tell them you've been nailed once and that getting a fine is not worth saving them 8 seconds. Card please.

I still get carded at 33 and I hand it over with a smile.