While I definitely haven't been training 20 years, I've recently been trying to build up my forearms. I was doing a lot of forearm work and grip training and I definitely started seeing results, so I stopped "overdoing it" because I don't see them as one of my weak points now.
However, I do experience a similar forearm pain that you describe, but on and off. I'm a software developer, and I've been on a computer for ages without any wrist/arm etc pain. Recently, while I'm at work typing away, I've started to notice my upper/outer fore arm towards my elbow start to hurt. It's a strange uncomfortable feeling... not like sore from training it or something. My wrists have also started doing the same thing. My work environment is no different and I wouldn't expect my arms or wrists or anything to magically start hurting overnight from nothing.
This is all occurring after I've added size to my forearms and strength to my grip. As I stated, I no longer train them like a weak point, so it's only ever touched on while doing bicep workouts 1-2 times a week, or when my grip is being tested by dead lifting once a week. I didn't experience any issues with this when I was hitting my forearms and grip hard, which is strange if I am now.
I'm curious to see if its a similar problem and if anyone else has any ideas. Fingers crossed!