the key word is "KNOW IT WELL" it does not matter the age or sex or else, as long as it is very well known to you or family member you can call them TU.

of course, I see it every day less and less usage. I have met folks in Madrid recently of my age bracket and call me TU, just because we are part of a group with same interests. My personal reaction is will never call you TU until I know you well.
but I am a traditionalist ::)