Answer:
7 years old
Step-by-step explanation:
Robert's mother pours a cup of milk for him and realizes that cup has a small crack so she pour the milk into another cup. Robert is not upset because he knows that the amount of milk has remained same.
As Robert is 7 year old . He can groups the milk in the cup according to size , shape and color. He has a better understanding of numbers and can understand problems easily.