Out of curiousity, does anyone know what the average age to have a first child is?

My husband and I were both 23 when our son was born and are now 24 with #2 about 3 weeks away. Whenever we go to the OBGYN I feel like I am the youngest person in the room. However, we were married for both pregnancies and were both out of college with full time jobs. Does anyone know what the "national average" is? I tried looking on-line but didn't really find anything.