Who needs to be on the lease

[Yashar Fred]

My tenants are two adults (unmarried couple) and a minor, 

Should the minor be on the lease agreement too?

How much should/can I inquire about the minor?

[Jake Wiley]

You should know the name of any human occupying the property and they should be listed on the lease.   You don't need to drill into the facts and circumstances of the minor, but you should have a legal name on the lease.   This actually protects the tenants as well as you, as you are only allowing those named on the lease to occupy the premises.   If the minor is not named, you have the right to say he can't live there, which would put the adults in a bind, thus protecting them.    It also protects you in that you can ensure that some strange situation isn't happening where unknown people aren't moving in and out of the property.   If that is happening you can terminate the lease and get the people out of your property.