A Cockapoo dog is a small dog so the Cockapoo tends to be fully grown by 6 to 7 months of age and then the Cockapoo lives an average of 14 to 18 years.
So once your Cockapoo reaches 6 months of age then it's really considered to be a fully grown Cockapoo because by 6 months of age the Cockapoo will likely not grow any bigger.
However some Cockapoos that are larger may not be fully grown until around 9 months of age.