Hindi - changing from Aap to Tum

Where is "tum" used and where "aap", to mean "you"? Can "tum" ever be used towards people older than yourself? Have you ever switched from using the "Aap" form to using the "Tum" form as the relationship with another person became less formal?

"Tum" तुम is used for people younger than or equal to the age of the speaker. "Aap" आप is used for people of older age, or even for people with equal or younger age when one wants to show respect/reverence/affection. For example, a mother can say "aap" to her small kid also with love. But these are informal instances. 

Changing from "aap" to "tum" as the relationship goes more informal, can also happen at times when used for people with almost equal age or status... It is possible and happens at times.