If the child has the social skills and development to be able to cope with it, then why not start them in school when they are old enough. I started school when i was 4 ½ (my birthday is in june) and i never remember finding it dificult to adjust or interact with the older kids - some of whom were almost 2 years older than me. I did go to pre school and child care though, so i had a lot of time with other kids before i started school.
On the other hand DH was a may baby and also started at 4 y.o but his school made him repeat a year.
Although, i dont have chldren so i probably dont know what im talking about.