Most nurserys take children from 3 months old up until school age. Playgroups tend to be 2 1/2 years or 2 years 9 months up until school age.
When your child is due to start school you will receive a letter from your local education body (part of council) which will tell you how to apply and have a unique code for you to apply online. You usually receive this in the November before they start, you have to apply by January, will receive your child's place middle of April and then they start in September.
My sons an April 2009 birth, had my letter last month, I've applied and I find out his school the day after his birthday. He then starts in Sept 13.
The way to think about it is school years run Sept to sept. So your child starts the Sept after their 4th bday but the cut off date is 31st august so if baby was born 31st Aug 09 they start school in Sept 13 but if baby was born on or after 1st Sept 09 they won't start until Sept 14.