Try to be funny about it, I guess. it's hard to come up with something on the spot (like when I was pregnant and in line at Wendy's and someone said I didn't need to eat, it looked like I'd swallowed a watermelon already--I couldn't think of anything to say, much less something graceful and witty)
If they ask if you enjoy being a grandma, you could say, "I'm sure I will, when the time comes." It's not rude, but puts them in their place.
If they're really bugging you and ask if your the grandma, say, "no, I'm their mom. Are those your grandkids with you?"
Age is all about how you present yourself. My MIL had her last baby when she was 41, (that was 16 years ago) but never died her hair until recently. My FIL's hair went white prematurely, and they really just don't care about how they look. They wear "old people clothes" and laughed it off when people thought their son was their grandson. it would have been enough for me to at least dye my hair!
This is mostly unrelated, but I was attending a class, and the woman presenting the lesson was dressed in boring clothes. She did little with hair and makeup, and she slouched as she stood. I was SHOCKED when I found out she's only 35! I would have guessed closer to 50! I vowed to dress cute and do my hair so I don't look that way in 5 years!
So keep up the good work, dress cute, do your hair (well, as much as most other moms do) and act like you are a 30-something. And keep a quip at the ready. Don't let them get to you!