It is fairly common for rental agreements to provide for additional rent if the unit is to be occupied by persons in addition to the named renters. I do not think the fact that a minor cannot enter into a contract has anything to do with it.
The contract is between the two adult renters and the landlord. If the lease provision is enforceable and you want to continue to live in the unit, I think you need to obtain the Owner’s consent and pay for the extra person the sum set out in Section L.
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The fact that the new tenant is a minor has nothing to do with this.
The dispute is between you and the landlord.
If the lease allows up to four people, and there are four people or less, then the landlord is probably not allowed to increase the rent.
To be sure, you should take the lease agreement any other documentation to an attorney for a consultation.
You could also contact the rent control board, if any, for your city.
There are also organizations that provide low or no cost help to people in your situation.
You can also read up on your rights as a tenant at the Department of Consumer Affairs website: