As you get older/more experienced, it becomes harder to find jobs. When "senior developer" means 5-7 years of experience, what do you do with 30 years? It gets hard to find places that will pay for that experience.
I've found, though, that there are jobs. You won't be switching jobs every 1-2 years - more like every 5-10. "Big engineering" places seem to value the experience more than, say, web places. (This may be just because I'm in embedded systems - there's not much use for that in web services.)
Your advantage isn't how fast you can crank out lines of code. It's how fast you can deliver real value to the business (what else are they paying you for)? Along those lines, one of the most valuable things is the bugs you don't write and the bad design decisions you don't make.