Minimum legal age to sign a rental lease contract.

Minimum legal age to sign a rental lease contract.
What is the statute in Missouri for the legal minimum age of a person to sign a lease to rent an apartment and be fully responsible? I have a 17yr. who signed and 1-1/2mo later turned 18yrs. He stayed for 5 mos & split. He now says he was under age to sign the contract and is not responsible for the back rent. The lease doesn’t specify a minimum age limit..
Your help is appreciated! Thanks