DS was 3.5 when we started him in preschool - I wanted him to be potty trained and I needed for him to be talking before I released him into the big world. He went for half days. I am trying to think back and we did a lot of community based activities at that age:
Mighty Movers Dance (This was actually the City Ballet group)
Mommy and Me swimming aerobics
Lots of trips to zoo and museums
Playground tours around the city using maps - he would tell me the ones he wanted to visit.
All of his activities were social and they were intended to get him to boost self awareness of body and space...he has been very athletic and we were trying to prompt him to have great reflexes - the social aspect was the ancillary benefit of what we were trying to expose him to.
You know your son - better than anyone else - if you see him shying away when social opportunities present, you might want to wait a bit longer - say until he has had some social successes with you being a part of the events so you can effectively assess his progress and evolution while giving him the encouragement and support he needs.
dx'd high fsh at age 30
ttc 6 years
5 failed clomid cycles
2 failed IVF cycles
#3 IVF/ICSI worked out fine
FET 2 years later didn't work out
Proud mom of an amazing miracle boy.