If you mean socializing in that they play WITH each other - well, that won't be for a couple of years minimum. Play dates with infants is more for the parent if you ask me. Babies can "socialize" at any age, just expect infants to not pay much attention to each other.
I disagree with PP because Riley is 6 months and two of my friends have LO's around her age, one is 5.5 months one is 7.5 months, and we all get together with the babies once a week and and all the babies hang out on the floor together and they interact with each other. They will watch each other play with toys and laugh at each other! I think the sooner the better! My SIL was a SAHM and when my niece got to kindergarten she had a very hard time adjusting to other kids because she never was around anyone else besides mommy and daddy.
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I don't know if there is a hard and fast rule about when they "should" start socializing. However, my DS is 8 months and he definitely interacts a lot with the other babies at his daycare. They move toys back and forth together, take toys from each other, smile and coo at each other, etc. It's really cute and his face lights up when I put him on the floor near his "friends" in the morning at daycare. I figure it's a good idea to socialize them at some point because it will make separation easier down the road.
It is up to you but I am a SAHM and I have been going on playdates with other moms who have kids the same age LO since he was 3mths. Just now they are really starting to interact (passing toys, staring at each other). It is up to you.
My DD loves her cousin who is a month older than her. She definetly recognizes her and enjoys "playing" (passing toys back and forth, crawling together, "talking",) with her.
Re: When should babies start socializing with other babies?