At what age should you let your daughter wear makeup?

Our daughters watch us put on make up and are persistent to use some too. But personally I don't think it wise for a little girl to wear makeup. I too, only allow it for school functions. My daughter pesters me to let her use my lipstick and nail polish so my standard answer is "when your thumb is as big as mine you can use them" So she comes and measures her thumb with mine every few days!!!