Grownup and Of age

Grownup adjective - (of animals) fully developed.

Of age and grownup are semantically related. in developed topic. In some cases you can use "Of age" instead an adjective "Grownup", when it comes to topics like adult, fully grown, nubile.

Of age adjective - Having attained a specific age; (`aged' is pronounced as one syllable).

Grownup and of age are semantically related. You can use "Grownup" instead an adjective phrase "Of age".