I'm trying to understand more fully the dynamic of using tum. Firstly, what kind of relationships do you use "tum" for? 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? What triggers this switch? Does the other person tell you to stop using "Aap" or do you just make the assumption yourself? I realise that in Urdu this may be a bit more rigid than in Hindi. So both perspectives are welcome.