>I may be generalising but I find older (40+) programmers more likely to write good code

[...]

>For example, in Common Lisp

Technology choice can correlate with age. In Lisp's case I would expect that it's long past "cool", i.e. that it's attracting fewer people than it used to, and so I would expect it to skew older.

Just like perl and tcl and awk.

Also you would have to take survivor bias into account - if you only see good lisp projects, maybe that's because the bad lisp projects died out? Maybe the bad old lisp programmers left?