I’m a children’s librarian. I’ve had baby storytime attendees as young as 3 or 4 weeks. A good baby storytime is about teaching the adult songs, rhymes, and fingerplays that you can do with your baby at home. You’ll appreciate it when you’re stuck on an airplane, in a doctor’s waiting room, etc. The social aspect is great for the adults too. The babies enjoy it, especially by 6 months, but it’s really for the adults.