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

If you are referring to formal teaching on numbers and letters, I would say 2 years old is the best time to teach first sounds and number 1-10. You can do alphabet and number puzzle. Melissa and doug brand has very good quality puzzles on these. For letters, you can associate it with the first sound of objects, e.g. A is for apple, astronaut and etc. I introduce an alphabet for about 1 week then proceed to the next one. I will do art and craft related to the alphabet of the week. Most importantly, try to make all the learning fun and interactive so they will gain interests and learn better.