That isn't true at all. Here is your free lesson in contract law for this week.
Elements of a valid contract are:
1. Offer
2. Acceptance
3. Consideration (ie what you are contracting for. Property, services, etc)
4. Legality of subject matter (The Consideration has to be legal for the contract to be valid.)
5. Contractual Capacity (If you were in a coma or drunk or mentally challanged, then the contract isn't valid)
6. Contractual Intent (If you thought you were agreeing to mow the lawn and then someone added a bunch of stuff about painting the house and repaving the driveway, then its not valid. You have to have intended to agree to the terms. You can't just slide in stuff in tricky ways.)
The thing with the age would arise out of the Contractual Capacity most likely. In the United States anyone who is under 7 years old is considered too young to enter into any type of valid contract. A 'child of tender years' is anyone between 7 and 14. Capacity could be argued strongly in that age bracket, but not always is a contract void because they are that age. It is going to be based on the particulars of the contract in question. People between the age of puberty and the 'age of consent' (usually 18, but sometimes younger) are considered to be minors. Minors are usually not bound to many types of contracts, but there are some that they can be held to. If you get married at 16 then you are going to be bound to the terms. If you enlist in the military at 16 then you will be bound to it. Those are facts. Typically when a minor is going to enter into a contract that may be binding, an adult is required to guarantee the contract. Kind of like a co-signer. Interestingly enough, contract law is a state thing. A situation that a minor could be bound by contract in Nevada may not be the case in Texas. I would consult a local attorney before signing anything as a minor just so you can know if the contract will be void or voidable. (voidable means it isn't necesarily binding)
In a situation like what this guy is talking about, I don't know where a contract would even apply. If you are hired to do a job and you don't complete it, then you are going to be taken to court for failure to provide services as promised. If it were any different then people wouldn't hire teenagers to work for them. Besides, the minors would be asking adults to sign contracts. Not the other way around. If the contract says "I will detail your car and you will pay me $200 dollars" then it is the client that is doing the signing. That would be the type of contract that would likely be enforcable even if it was a minor signing. Other types of contracts involved with starting a business would likely require the signature of an adult to guarantee the deal in case the minor flakes out and breaks the deal.
That is your contract law lesson for the day.

Please join us next week when we discuss tortious interference in business law. :naughty Sign up forms will be available in the break room.