Yep, because the people that wise off don't get it, anyway (unless he knew you, then you owe him....)
Some comebacks, all with a chagrined smile:

"I'm too cheap to buy speed."
"When you grow up, you can get a grown-up bike, too."
"What are you compensating for, again?"
"You'll get laid more often with wine than Red Bull."
"Wow, nice ride; my bike costs less than your front wheel."
"Como esta, carbon mafia!"

Usually I just smile. Around here, they're runners, too, part of the tri-yup crowd. They usually know me and just wave.
Away from here, I've truly never had anyone really give me anything but a wave before, even the carbon commandos.

Then again, I'm often with a 25-year-old tri-chick, the daughter of a friend.
The poor lads are quite often trying to avoid running over their tongues.

If we stop, and the conversation gets around to their times (with triathletes, it always does), I just ask them their PR's.
They'll give me the times, paces, etc. I mention that it's nice they run at a "social pace," without any real pain involved.

There's a benefit to being old and fat. But there's also a benefit to having once been young and very fast.