well my little one was 6 weeks old when she started.
she is now 2 and loves school
i believe she has gained alot fo ocial skills and communication as they do alot of talking and reading and very interactive things at school which i prob wouldnt do at home.
she also learns what is acceptable for a child her age,
yes she gets a runny nose but nothing bad enough to keep her home. i think she gets sicker at home from the bugs that are on my uniform when i come home from work. lol

so far only negative is she doesnt want to come home of an afternoon so its a struggle at pick up time but not drop off time. lol