Usually kids can start solving suggested problems at 10-12 years, but it is not too late to do it at 13-14 years.

Can I start earlier?

If you think that you kid is interested in math or mathematically gifted you can start earlier and try

FUN ARITHMETIC

You can also watch on Youtube

How to get 100?

Fun Arithmetic.

For kids and parents who want to get ahead we recommend buying a book

It's not a joke: some of these problems often surrface during interviews for Software Development and Engineering positions.

A good example is

Youtube version:

Twelve rings,

Fake coin,

Four pearls.