Buy a fake moustache
Haha alright on a serious note - if what you offer is valuable enough, the client shouldn't need a reason to ask how old you are.
Be professional, courteous and confident; it is important to be confident above all. Do not swing yourself around as if arrogant, but show enough so that you do not appear as if you are a deer staring at approaching headlights or a virgin staring at a naked woman...or dude? for the first time.
Let's imagine that your age is asked, play it off - be witty about it.
"Haha you look rather young! How old are you?"
My response would be, "well, damn, I will take that as a compliment. How old do you think I am?
"Blah blah blah"
Then word it so you turn the tables around, but do not appear affected by that question as if it makes you uneasy. Such a reaction may turn the individual more curious about a subject that should be irrelevant towards adding value to their business.
Perhaps turn it around by saying, "speaking of age, how long has the business been in operation? How long have you been a business owner?"
Be the person asking the questions. The time spent shouldn't be about you unless it is relevant towards what sort of value you offer.