I know one guy reported improved smell after he put beta-NGF solution into his nostrils ( http://www.longecity...2272-ngf-spray/ ), and I have noticed improved smell at times when using Cerebrolysin ( I'm not sure if it is permanent or not , mostly notice it while I used it , but I'm young ).
I'd try Lions Mane since I did hear it stimulated NGF levels, so maybe that could help bring some youthful function back, who knows.
Perhaps Noopept which is another compound I recall hearing about stimulating NGF or BDNF. Noopept should be very safe just keep in mind it might affect blood clotting (as does a lot of things).
I'm no expert but smell does have to do with the neurons in your nose, as well as those in your brain.... and it isn't impossible to grow a few neurons (or dendrites) although who knows how useful it would be in all cases.