I found the following OSHA manual on hearing loss interesting and they even have a section on Conductive Hearing Loss (flat loss across the frequency spectrum) with graphic illustration:
https://www.osha.gov/dts/osta/otm/new_noise/#hearingloss
Wikipedia has an extensive listing on causes of conductive hearing loss (although Um_bongo’s post above leaves a flat sensorineural hearing loss in as a possibility):
If one does not like non-descriptive lists, it’s easy to find lots of Internet article on conductive hearing loss causes and possible remedies, e.g., 10 Possible Causes of Conductive Hearing Loss (emphasis at end on an audiologist or ENT exam to differentiate between sensorineural and conductive loss).
There’s a priori probability and posteriori probability. One has to be careful not to assign the former probabilities to the latter, i.e., someone shows up at age 39, who’s collapsed, has shortness of breath, pain in chest, etc., you don’t say, “There’s only a 1 in 100,000 chance it’s a heart attack, on to the next thing.” The presenting case changes the probabilities as to what might be involved.
A simpler example would be: “What are the chances I’m going to get a skin infection?” If you’ve scratched yourself, the answer is a lot higher probability than if not.
So here the presenting case is an unusual apparent conductive hearing loss at a young age.
If you already come from a family line with a history of hyperlipidemia/hypercholestemia (as my brother-in-law) does, the a priori chances of having bad outcomes at a very young age are not the same either: Hyperlipidemia: Symptoms, Causes, Treatment, and More
So we might be worrying the heck out of Johnmurphy here, but I imagine early onset of just about anything is a good excuse these days (now that more and more molecular genetic tools are becoming available) to look into whether family history factors might be coming into play in whatever is causing the presenting symptoms (while not disregarding possible environmental causes in the least).
I guess the important thing that Johnmurphy does not mention are what are his bone conductive losses, if any, across the frequency spectrum. Presumably his audi has done these and the results might be available. And maybe the HT audiogram tools should have a way to represent bone conduction results? (guess they could always be textually listed)