There is a saying my friend: Do you know what the difference is between a custom gun builder and a large pizza?
A large pizza will always be able to feed a family of four.
Attending the Shot Show, the largest industry sales event for modern weapons, for several years I have not seen any "unwritten" age limit or a glass ceiling. One of the most informative people I have talked too in the world of weapons was a female rep, who was quite attractive, and worked for AAC. Not only did she know all about their product line, like the coveted Black Box suppressor, but she knew all about the weapons it interfaced with, like my H & K USP Tactical.
That said, most all the booths are "manned", literally, but middle aged men who developed products for their particular company, whether self owned or as a team member. Qualifications are never discussed, like degrees or achievements, but most of the people that do really well as sales reps have a combination of real world ( military or L.E. ) experience and are very affable.
The world of modern weapons is FILLED opinion and "expertise". Making a large sale, especially one of something that isn't considered customary gear, will depend greatly on you and your presentation.
Having said that, here is something to whet your appetite. At the Shot Show in Feb. I was standing at the H & K booth, fondling the incrediable HK 417 that will never be legal here, and a general and two business "associates" in traditional Arab clothing came up to the booth, with their appointed rep, and began checking out the 416/417 series. The General was a big fan of H & K and I believe he was trying to encourage the purchase, as evidenced by his assisting the sales rep in discussing the finer points of the weapon systems. They were from the country of Jordan and they were discussing a sale of 100,000 battle rifles now with a contingent of another 100,000 should the weapon meet their "Standards".
At that point I merely wandered away cursing $3 plus dollars a gallon for gas...
Best of luck to you in the field if you choose to enter the fray.