ANSWER

11 - 12 year olds

Maria has 8 times the amount of water than Cosme

The volume of the Cosme tank is 1x1x0.5 = 0.5 cm3

​

Maria's tank volume is 2x2x1 = 4 cm3

4 = 0.5 + 0.5 + 0.5 +0.5 +0.5 +0.5 +0.5 +0.5.

In other words, Maria has 8 TIMES the amount of Cosme's water.