We are in Pa. Leasing Co requires all persons 18 or older to sign the lease. My son will be leaving for college in 2 months. He is not paying rent on this property. I am not familiar with this requirement and more than a little uncomfortable not knowing what the legal ramifications of this would be for him. Please advise

Then you are paying rent ? Whoever is renting property has to sign a lease. What do you mean he is not paying rent on the property? One has to guarantee they will pay for the property for a certain period. It is standard procedure.

If you are leasing or renting the property, and he is your child, living at home,
1. how did they know he turned 18 ?

From their records ?

What is the written rules in the lease, what the lease says, is all that matters

If they insist that he sign the lease, or they won't lease the property to you, your option is to not lease from them; go somewhere else.

First, Is this a new lease you are asking about or an existing lease? If an existing lease. When did it start and when does it expire? In either case, you need to read your lease. Is there anything in the lease that requires all adults to sign the lease? If it is not in the lease, then ask them to show you where this policy is written, I don't think its part of PA law.

Another thing to look at is HOW they want him to sign. Some landlords do require that occupants be listed to prevent the tenant from bringing in other people. In some cases, a person can be signed as an occupant, which does not make them liable for the rent. So you need to get some more details about this.