I'm mid-thirties and was recently asked to teach a 50 yo best practices for development in Node. And not like Node specific patterns, but more like how to handle http requests. I'm pretty sure that guy is way above my skill set - he's smart and he has 15 years of work experience on me. I turned the offer down since I would have looked like a fool, like a kid teaching an adult how to drive.

The age bias is real.

It sounds like you missed an opportunity there. Just because someone has XX years of work experience doesn't mean that they know a particular technology stack. Perhaps this could have been a great opportunity for you to teach that person your skillset, while learning things from them in return. The best mentorships are bidirectional.

While humans are growing up, we're accustomed to view our "elders" as our superiors in all respects. Childhood teachers encourage this view. However, at the point when you enter the workforce, in my opinion you should set these ideas aside and treat everyone as a peer; and keep your mind open to the idea that both people may have something to learn from the other.

No, I think I was a bit unclear. The guy obviously knew what he was doing, and had setup projects in Node. For me to come in and tell him how require() works would be insulting.

No missed opportunity, I pivoted the deal after turning down the first.

I don't think you were unclear, I think maybe GP only read the first two sentences.

If you become comfortable with annotation heavy code bases, what is the likelihood it will change in the future and you have to start over with a new style of codebases?

If there's a major shift in those languages (vs. JavaScript) it seems more worth it to retrain once it's solidified and you see companies actually looking for those skills.

Fast moving ecosystems means more things become deprecated more quickly and you also risk more "train thing that will be useless next year".

but not everything is moving into the could/servers, who does development for desktop nowdays? phoneapps also have same structure as webapps, what other type of dev you mean, industrial?

I tell people, look around. Think of every single electricity powered device you interact with or pass by in a day. Someone is doing development work on a lot of those. It's not always flashy work, but it's darn interesting work.