"Vraiment?" fit-il.

[alan gould]

Saw this in the dictionary,  translated as "he goes" for "he said."  Many people will use this in relaying a conversation, but it's very casual and adolescent and I never used it.  Is it similarly adolescent in French or more common?  

merci d'avance

[Vedas]

you'll see that rather in novels and it's quite formal actually, in French the casualness is not in the expression "fit-il"  itself but (depending on the context) implied by the attitude of the utterer :

he said indifferently/unconcerned. There might be some irony too.

You can use it :

- to avoid the (possible) repetition of "dit-il"

- to  show less involvement / less respect/ irony/innuendo in the discussion

As a rule of thumb anytime you come across  the inversion Verb + Subject, it is likely to be a formal expression.

[George Hunt]

I don't think it can be a similar usage because this example seems more likely to be  part of a written dialogue -and not a part of a direct conversation.

The tense is the 3rd person singular of the past historic and would never  ,to my knowledge be used in casual conversation.

That makes me think it is unlikely to be loose language but I have never come across it in normal conversation  (where it would presumably  be used as "fait-il."  or "il fait" -present tense .)