Kids And Cosmetics

I occasionally use cosmetics.. And it includes mostly a kajal n a light lipgloss/stick.. Recently my niece who is 5years old,was with us and she insisted that she wanted to use those cosmetics. How much ever I tried to distract her, it didn't work out and she wud come back to d same topic. I as a parent feel that children shud decide reg cosmetics when d appropriate age comes and not as a kindergarten. There r better things to do in that age. Her parents too don't approve of her using cosmetics, so to shut her up, I didn't want to resort to letting her use it lest it send d wrong signals to d kid especially.
So, my question is, when I ware lipstick in d presence of a child in a group of adults where no one does, the child starts asking me to extend/help her out with the same. How do I explain to the child that there is an age for everything, and this is not the right one! Have u been through similar experiences?? If it had been my own child, I would have been firm and held my foot down, but since it involves another set of parents too, I want to deal it diplomatically

P.s the kids parent got irritated after a point, when I no longer cud handle d discussion wisely, and shouted at d kid to stop the nonsense.. And now the kid starts whispering to me that "we shall talk abt our secret later aunty!!"
As an after thought, there are parents who have no issues with small kids using cosmetics, but since I have and her parents have, all this discussion