I introduced at the age of 3 , he use to do simple 3 piece puzzles. Now he is 6 able to do puzzles with many pieces. I bought a few from Wondrbox. It's very good

After 3 i guess, best time to introduce small puzzle, check Wondrbox . You will get as per age and pocket friendly budget. Really good one.

Start with2.5 yrs age

You can try giving small puzzles with age 3.5 . Start with less number of pieces and then gradually increase it. Try wondrbox, they have puzzles for all age groups.