I'll let my son to study pre-nursery when he's 2yrs old. As he's the only child at home, he's quite bored even he has many toys and my mother plays with him lot.
I hope he can have a better way to learn from playing, and starts to know what's discipline and social life. I believe the school can provide an environment and have the skills to teach my son on those.
It's not must to study pre-nursery. My colleague's first daughter did but her second daughter didn't. The mother said the big sister was bored at home so she starts school at 2yrs old. However, when the little sister was born, she can play with the big sister so she didn't start school so early.
I think you can think about your own case and see if you want your kid start school earlier.