Understanding Contractual Capacity - The Bhai's Dhaba Analogy

You know our street food scene, right? So, in a contract, a person (let's say a bhai running a dhaba) needs to have capacity to enter into a deal. Imagine bhai's dhaba and he wants to lease the space from the owner. Simple, but what if bhai had just 15 years and 6 months? Or what if he was a minor (in IPC it's 18, but in Contract Act, 21 is the age)? Dhaba's owner (the other party) might think bhai is old enough, but legally, bhai can't enter into a contract. Same with contractual capacity - if the other party knows (or should have known) that bhai is below the specified age or mentally unfit, then the contract's null ( invalid).