I have tried to get The Jet Age and specifically The Jet Age for around 2 months.
You cannot just get 'The Jet Age' as a stand alone module unfortunately it has to be part of a larger install because all the modded stuff here requires a 4.12 game version first and foremost, any jet based mods are added into the existing game and require it to run at all.
You can add ModAct 5.30 to your existing 4.12.2:
https://www.sas1946.com/main/index.php/board,284.0.htmlThis will then allow you to build a jet package by adding in this as a start:
https://www.sas1946.com/main/index.php/topic,15649.0.htmlThis is the basis for anything jet related that has come since, however, besides that there are various other things involved but that will at least get you going.
The best overall way is to use a mod pack, there is the VP modpack that is a WWII package but does have a separate add on for jets:
https://www.sas1946.com/main/index.php/board,286.0.htmlB.A.T. is the most up to date way but you will have to download and install this first of all, 'B.A.T. v4.0 "RED CORE" - The World At War pack':
https://www.sas1946.com/main/index.php/topic,65026.0.htmlThe other three modules one of which is for jets, come as another addition that can then be downloaded and added later if you want to expand to the full pack and have access to jets.
Hope I got all that right, I am easily confused, that shows the various ways as far as I can remember, as you can see none are a stand alone jet package.
Take care and be safe.
Wishing you all the very best, Pete.