My DD started at 21 months - and was fully day trained by 23 months and was doing similar things to your DD. I guess the only other criteria I went by is that she was dry for at least an hour at a time, usually longer, so I knew that she was able to hold on.

No harm in giving her the opportunity... we're getting into good weather to just toddle around with no nappy on and see if you can catch those wees!