Generally from the age of 5 years...also always go for offline classes as they are too small to concentrate on the online classes.

Abacus training focuses on the concentration power of your child. Concentration plays a very important role in learning anything. It is believed that 4 to 7 years is the best age because they adapt early whatever is been taught to them. Also they are not loaded with too many responsibilities or you can say they are just like a blank paper, so before you allow something to be written on it make sure it is effective, knowledgeable and would help them in future:) and It would be teach abacus offline as child doesn't take proper attention which is required for abacus.:)