I have a child's voice what can I do?

I'm 20yrs old and everytime I'm on the phone or call for a job offer whether internship or summer job I get asked if I'm even old enough to work and get laughed off when I say I'm 20. It is a really frustrating situation. I'm a young high school graduate, I already have a bachelor's degree and am starting law school this upcoming fall, yet nobody takes me seriously. I don't have a childlike speech, I like to think I'm articulate and don't lack vocabulary. I will admit I'm naturally high pitched as I've been in choir and have been able to reach really high notes that my fellow students could not. So what can I do? I try speaking in a deep tone yet, that doesn't seem to help. When I order pizza I always get asked if I'm old enough to be using a credit card or ifits my mom's card. This situation has become really frustrating, on top of that when people see me they swear I'm 16 which doesn't help things. I constantly get carded at movie theaters etc... Anybody else with this problem? How do you deal with it?