My daughter was 3 months old when she started going to family day care 2 days a week, and then she was 4.5 months old when she started going full-time (because I had to go back to work).
Up until now (she's 18 months) she's always been the youngest at her FDC and I find that she has really benifitted from it. She started counting to 10 at about 14 months old and she's usually played well with other kids and shared well. She's very social and confident. I don't think she would have been as social and confident if she hadn't been to daycare from a young age.