How do I deal with clients who are concerned with my age and not experience?
There's no way around it, people will always initially judge you by their first impression of you. So, make that first impression as polished and professional as possible. This will give you the opportunity to hold them long enough to hear what you have to say. Once you do speak, avoid youthful slang and make strong eye contact. Beyond that, don't overcompensate, just remain professional and demonstrate a positive, upbeat attitude.