Yeah, it's after puberty -- but are you so sure it would be hard to miss? Small effects with lots of confounding variables and an enormous profit incentive to miss seem easy to miss.
In order to do due diligence for this reply, I typed "finasteride" into Google Scholar, and the first suggestion was "finasteride side effects", which brings up 20,500 results, including highly cited papers dealing specifically with sexual side effects:
https://www.sciencedirect.com/science/article/pii/S174360951...
https://www.sciencedirect.com/science/article/abs/pii/S17436...
https://journals.sagepub.com/doi/full/10.1177/15579883145384...
https://www.sciencedirect.com/science/article/pii/S001502821...
I think that in that context, it would be hard to miss!