I was 'late' to be night dry, around 7 so was all prepared for a long haul but DD1 decided to go into undies just after 3 and was surprisingly fine. I wouldn't have worried till 7 though, I consider 3 or 4 early lol but that's looking through the eyes of my personal experience.

As long as it isn't a big deal to the kid then it isn't a big deal. When it becomes a big deal for them (it never bothered me, I remember wearing nappys to school sleepovers, it seemed normal to me then too, I'd recall the memory and others would think it weird, I'd never thought to till then lol but it can come to worry others), look further then and work it out together, let them have the ownership of it.