My grandmother makes up ridiculous answers like, "28! Don't I look young for my age?". It usually gets people laughing.
My mom likes to haul me over, give me a hug and say, "Old enough to have a beautiful daughter like her!" (she's even done this with a friend of mine, who looks very much like us!) OP might be able to adapt this to something like, "old enough to have a 13 year old son!".
I personally like saying things like "I have no idea. Last time I told someone my age I was off by two years. I'd have to go figure it out". It's actually kind of true - I'm very bad at remembering how old I actually am and I have told people the wrong age a couple of times. I also say "Old enough to drink" or "Probably older than you think" if I don't mind telling someone.
Someone guessed my age last week at work, and was off by 5 years. They thought I was at the most 28, when I'll be 33 this year. I told her my actual age, and her jaw dropped. She said, "but you look so much younger!". My response was, "Oh, well you didn't tell me you were guessing maturity instead of age! I'm like 12 or 13 by that standard!".