Gosh... I completely understand this dilemma! I actually have a bit of experience and I STILL struggle with this, even 11 years later. People think I look twelve AND being a women in construction might be just as bad. There's a few things a do:
1. Try not to care. At all. If someone is really concerned about your age they are a know-it-all and why would you want to work with someone who doesn't respect you based on age alone? Move on to another relationship, you'll be happier and thankful you did.
2. When someone has enough balls to ask your age (let's be honest, the question alone is them trying to insult you), I tell them I'm happy to reveal my youth if they are willing reveal how old they are first.
3. Be confident, if age is brought up turn it positive. You're young, motivated, educated and have a fresh outlook bringing new ideas and creative value! I'd rather work with a person with their ears and mind open than a person who is a know-it-all blinded by age and not open to opportunities.
Bottom line, don't care and be confident and proud of your age and where you're at. Anyone who can't respect you doesn't deserve the value you bring!