If a minor enters a contract, the contract is not invalid, it is voidable. That means the minor or their guardian can at any time void the contract (in other words, the contract doesn't exist anymore); until that happens, the contract is perfectly valid.
Your friend, assuming he or she is an adult, cannot void the contract. The contract is valid until you or your guardian changes their mind about it. For your friend, the contract is binding. For you, it's not binding as soon as you decide that you don't want the contract.
As Dale said, minors or their guardians can void a contract until the minor isn't a minor anymore (and for a reasonable short time afterwards).
So if you are a minor, that shouldn't stop you from entering the contract. Of course you should make sure that you fully understand the contract; you get protection by the law because the law assumes that you might not fully understand the consequences of the contract and get ripped off as an unexperienced person.
If your friend is a minor, you need to determine how much you trust your friend, how much he behaves like an adult, and what's the worst that can happen. For example if you inherited $50,000 and invest it all in the business, the worst that can happen could be that you lose it all. On the other hand, if you only invest your hard work to get the business up and running, worst case is that you lose the work that you invested.
Of course if it is a person that cannot be trusted, then it doesn't matter how old they are, because they cannot be trusted.