I think the key(s), whether you're sixteen or sixty, are natural intelligence and a great work ethic.

I'm fifty and am full-stack in whatevers with a side of devops/architecting. I frequently have to walk into a room and in less than thirty minutes figure out what the hell is going on, how I'm going to add value, communicate that to the stakeholders and then deliver. People seem to dig it.

Fifty ain't so bad. What's COBOL?

FORTRAN, on the other hand, is used to melt compute clusters while trying to make things go boom, or model micro- or macroscopic properties to design various things. Damned language still optimizes or runs large scale better.

As you all know, few pay to rewrite code that works well.

Age has nothing to do with it, relevancy does, and some domain expertise is difficult to acquire without decades of experience, even on the bleeding edge.

Unpopular opinion: Silicon Valley is not the center of the universe.