I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
Usually, the age of binding contracts is age of majority = 18. As a minor, you can disaffirm a contract -- like a child actor can disaffirm his acting contract.
Now, in your state, age of majority (especially for wills, etc and all elsewhere) seems to be 18. However, "adult" means 21 years or over.
For a binding contract -- 18 unless i am assuming it has to do with alcohol or gambling.
hope this helps.