The baseline characteristics of these fracture populations showed closely resembling median age, sex and comorbidity distributions.
This is definitely incorrect. "Resemble" is a transitive verb, and while listing several things as the subject does allow the reader to infer that they are each others' objects (that is, they resemble each other), it's not grammatically correct, and is very awkward. Also, in this wording, it's the median age, sex, and comorbidity distributions that are doing the resembling, rather the fracture populations.
The baseline characteristics of these fracture populations showed a close resemblance in median age, sex and comorbidity distributions.
This also doesn't actually have the fracture populations as the subject of "showed". Rather, the subject is "baseline characteristics". The structure of this sentence is rather odd; you are saying the characteristics are similar in age, etc., when actually it's the populations that are similar. I recommend "The baseline characteristics of median age, sex, and comorbidity distributions were very similar between these fracture populations." Perhaps replace "between" with "across", although that might be misinterpreted as "within".
As a side note, I find the term "fracture population" highly opaque (does it mean "population of people with bone fractures"?). Perhaps the context provides more clarity, but if you're using it on its own, you should be more explicit.