I was watching Folge 03 and they said:
Ab wann kann man denn Talent erkennen?
I understood the question, but what is denn doing here?
In questions 'denn' is often a matter of emphasizing, suggesting a nuance of scepticism or reservation. In your case the question might suggest doubt that talent can be recognised at all.
A nearly synonym sentence would be "Ab wann kann man Talent überhaupt erkennen?" A proper translation to english might be "When can we recognize talent at all?"
This is one of the elusive German Abtönungspartikeln. If you use it in a question, the implication is that you are questioning a subtly more general point than the one that was under discussion before. Example:
Ich finde Cindy ziemlich talentlos. = I think Cindy isn't very talented.
This is a statement about an individual.
Und Bert? = What about Bert?
This is a question about another individual.
But with denn:
Woran erkennt man denn Talent? = What is talent, anyway?
This moves the discussion in a more general direction.
I believe (and I am a native speaker) that denn here indicates that the speaker considers the fact that after some point talent can be recognised as a given. (Also it seems more natural if there has been a previous utterance to that effect.) The same utterance without denn sounds a lot less connected with what has been said before (It could be the title of a book, for instance), while with denn it seems to relate to the context.