If the model has a release form signed, is it legal?
In the US you must be 18 to buy sell or control property. You may have a guardian do business on your behalf.
As you mentioned photographing models, you need to be careful that they are not exposing any vital parts or you could face charges for being under age.
Since a minor cannot enter into a legally binding contract with another person no, it is not legal. A guardian/parent would have to be involved.
only with the models permission.
Yes
