She may want to start training during the day but I'd keep the nappy on at night for now in case it's just a phase and then she won't get distressed if she wakes up soaked one morning.
I know some 18mo's seem ready for TT, the only thing is that it can be so difficult to maintain because they're short and often can't dress/undress themselves. So TTing becomes a series of "emergencies" where you have to drop and run.

Still, if she's interested, give her a couple of tries at least. Briona was interested at 18mos, I think she just wanted to copy Hannah, so we let her sit on the toilet until her facsination passed. Now she's just starting to TT #2's and the neat thing is that she is already comfortable in sitting on the toilet.