When you first go to a park ticket booth to activate the passes, they usually require IDs from all adults (18 or over) and the IDs need to show the same address as the member. They usually do not require IDs from children under 18.
As noted there is no age limit to getting the pass, just the same residence limit and thus if a grandparent is living with the member, the member can purchase the discounted pass for that grandparent. However, there has in the past been an eight total family limit.