I'll let Madge give you RCI offical policy, but many resorts require the guest at check-in to be the age of majority. So in the US, that 21. In Canada it is 19 and in Mexico it is 18. RCI offical policy may be different.
As far as damage, when anyone check-in, they give a credit card which should cover any problems (damage) but if the CC doesn't cover all the charges the resort may go after you. ( very unlikely)