My baby is now 6 months old. Is there any restriction for us to rent a 1 bedroom apartment for the net year or so? Is there any law regarding the baby's age when we rent an apartment in PA?

The only laws that apply in this situation usually apply to public housing. So, unless you are applying for subsidized housing this shouldn't be an issue. If money is an issue, I could see keeping the child in the same room until they are out of a crib.