It seems rather impossible to me that someone would voluntarily continue programming for a couple of decades and not be excellent.
I don't have any hard data I'm afraid - just my fallible recollections ;-)
Some of ways of getting non-excellent old developers that I've observed:
* Not all old developers have been programming for a couple of decades. People can and do come into development late - and suffer all the normal problems of newbie developers.
* You'll be amazed at how little work you can get away with in some large organisations. When you have a couple of hundred people on a project you will find one or two Wallys from Dilbert.
* The devs who have sunk deep into some gnarly legacy system or language. Being the person who knows the right bit to tweak in the middle of a 1500 line procedure in the middle of a big-ball-of-mud project might be stupidly valuable to a company - but produce a lousy developer in any other context.
* The "senior" developer / architect / lead who has been Peter Principled to their level of incompetence, but whose team is good enough to cover up the deficit in leadership ability.
* The large chunk of bad developers (of all ages) who don't realise they're bad developers. Folk can't improve until they understand where they suck.
I don't have any hard data I'm afraid - just my fallible recollections ;-)
Some of ways of getting non-excellent old developers that I've observed:
* Not all old developers have been programming for a couple of decades. People can and do come into development late - and suffer all the normal problems of newbie developers.
* You'll be amazed at how little work you can get away with in some large organisations. When you have a couple of hundred people on a project you will find one or two Wallys from Dilbert.
* The devs who have sunk deep into some gnarly legacy system or language. Being the person who knows the right bit to tweak in the middle of a 1500 line procedure in the middle of a big-ball-of-mud project might be stupidly valuable to a company - but produce a lousy developer in any other context.
* The "senior" developer / architect / lead who has been Peter Principled to their level of incompetence, but whose team is good enough to cover up the deficit in leadership ability.
* The large chunk of bad developers (of all ages) who don't realise they're bad developers. Folk can't improve until they understand where they suck.