FR: être enfant - absence d'article

[Garysfraser]

In this sentence:
Lorsque j’étais enfant je disais que j’avais mille un amis.
why are there no articl

 

[Maître Capello]

There is already a determiner before amis, namely a number (mille un). No article can be used in this case, like in English. (You don't say ten the friends but simply ten friends.)

Regarding être followed by a predicative noun without article, see:
FR: être (le) vassal du roi - article défini
FR: être (un) anglais - article
FR: être roi / être un héros - article
FR: être/devenir (un) esclave - article
FR: être/devenir (un/une) + profession/fonction - article ?

 

[Gérard Napalinex]

And if your question is about enfant, the absence of article is due to the fact that it is used as an adjective meaning young.
You'd see this in some pictures titles, such as "Mozart enfant" by Cignaroli
(and for some reason mille et un is more idiomatic than mille un)