When should I start teaching my baby numbers and letters? So far we're doing nursery rhymes.

Yes. There is no rule to when a baby can learn things! The earlier the better. They may not be able to fully understand what we are teaching now but eventually they will learn it. I started teaching my boy numbers and alphabet using fingers and flash cards since he was 3 months old. I use my fingers to teach him counting of one to ten. I will also do addition by using my fingers. My boy loves it and will be very attentive whenever I start counting. He loves flash cards too. You can get these flash cards from Early Learning Centre at United Square.