Every 5 years on average since the late 90s the industry has doubled in size. Add in natural attrition (and the other things you mentioned, ageism, management or other tech adjecent careers, etc.), and even accounting for a modest number of "second career devs" starting later in life rather than out of college, you still have an industry that skews younger simply by virtue of overall growth patterns.

I think that is significantly overlooked when people ask "where are the 50+ engineers?".