found out coworker/new friend is 48 and thought she was around 30!!

JK
Have you ever had one of those moments when meeting someone new that they turned out to be a lot older than you thought? I just found out a coworker I've been hanging out with a lot is 48. I'm 27 and I was thinking she was more around her 30s. I don't care too much that she is a lot older, it just shocked me when it was revealed her age. She was talking about how it was amazing growing up in the 60s and 70s, that's how I realized it. Then she told me too she is 48. I told her how amazing she looks for her age! I wish I could post a pic of her but obviously can't. She also acts way younger... whenever we went out for drinks she would be the craziest. The guy she is seeing looks in his 30s too which may be the case. I don't know...when I did find out her age I was a little put off the way she acts so young. Just still shocked. 
How about everyone else?