quand il était bébé

Hi Kathleen,

I am not aware of any particular grammatical rule but it is the same as with jobs where you don't use an article before the noun to describe the state.

It is the same with 'devenir' which is used in the title - Devenir tata ( Becoming an auntie)

and 'devenir papa' at the end of the exercise( to become a father).

So you will have -

être/devenir maman, être devenir oncle/tante, être fillette, être/devenir adolescent ( to be/to become a teenager) etc.

I hope this helps!