Is 15 months too early to start training Faith?

The thing is, I think she understands where wee comes from.

When I go to the loo I tell her that mommy is making a wee, then she will clutch at her nappy and now she has started squatting and when I am finished she stands up and starts clapping.

Is it too early?  Should I wait? How would I start training a girl?  Aaron was easy, we made him wee in the garden, no mess, no fuss…