I used flash cards with my kid when she he was around 15 months. I introduced him to the numbers in a playful way, sometimes singing the number holding the flashcard at the same time. And since he picked it up fast, it became a family game. Even though, we started at 15 months, I believe if the kid is showing interest then one should continue, else no pressure should be put on kid to learn numbers at this age. If not at 15, then at 2 or 3 or 4, the kid will eventually pick up numbers, alphabets and all.
I start both my kiddos really early around 6 months. Mostly just flash cards. Normally, when there is a chance I will just point out things to them, like for example "look ! You have 3 balls" "there are 2 dogs over there" gradually they will understand without much difficulty. My daughter is now 3 years old, she can do simple addition. Most importantly is to make it fun!
A baby's brain is 80% as developed as an adult. Teach him/her anything and they can absorb it. Use flash cards, colorful blocks, write on a board etc...I used to play a game with my kid where I would draw and make him add it up....he used to love that a lot
anytime! it can even start when they are like 1-2 yr old.. keep repeating is the method i used on my kiddos.. :P
Rajan Gupta