Thank you for all of the very good responses!
I always thought my "old enough" answer was bordering on rude because it could be snarky(?).
A few years ago, my company hired someone for my department. When a coworker and I first met this new employee, my coworker pointed out how young this person looked and asked her age. New employee responded with "old enough" and that was the first time I had heard it. At the time, I thought it was a snarky answer, but I changed my opinion later on.
I haven't been able to pin point why the convenience store cashier situation bothered me as much as it did. I am not ashamed of being a young mother, but I acted as if I were. I wanted to drive back and give him my real age, but I would just look crazy.
I haven't been back to the convenience store yet. I had been going every day before the gym for a very large water, but I haven't wanted to go back. Maybe I'll go back in the next few days just to make myself feel better.
Thanks for the great advice, everyone!