Don't tell them your age, then. Age discrimination is illegal so they can't ask how old you are and you don't have to tell them.
I was contacted by a well-known men's magazine when I was in my teens regarding some blog/Usenet posts I'd written. They wanted me to go to a conference in the US and cover a story for them. The initial e-mail thread went very well then I made the mistake of admitting I was 16 on the first call and never heard back from them.
Covering a conference they'd eventually have figured it out - but with dev work the client need never know your age and as long as you are legal to work, they need never know.
This isn't legal advice, but as someone who worked when I was a teenager, parent consent may be required, so then they'd know you are a minor.
Under US Federal law, it is illegal to work under the age of 14. 14 & 15 can work but only during non-school hours. 16 and older are fine, but under the age of 18, parental consent is required.
You have to tell them your age at some point. In Germany there are restrictions in place to protect minors disallowing them to do certain kinds of works and restricting when and how long they are allowed to work. You really don't want to break those laws and not just due to legal repercussions. I would be very much surprised if the situation would be significantly different in other (civilized/industrialized) nations.
It is illegal to discriminate against someone due to their age, but it isn't illegal at all to ask. Most places just avoid asking so noone can accuse them in the first place.
