Yeah I think May. Tis one of those 'mother (usually) knows best' issues really, although their childcare/kinder teachers are a great resource for deciding too obviously, getting to see your kids in a school-like situation.

I think the 'hold back' school of thought comes into play if you're really unsure, for a variety of developmental and social reasons - if in doubt, hold them back, because it's less likely to cause problems.