Then again, I think some of it is subconscious and unintentional: unreasonably harsh age-grading, coming from an inexperienced and privileged set of people who've never had to do grunt work and therefore have slack-free careers. People who are young and have high career efficiency ratios expect a linear trend-- consistently improving projects-- but the reality for most people (save privileged 22-year-olds) is that they have to take the work they get, rather than being able to focus on their passion or highly-visible (e.g. open source) work. I, personally, would love to hire someone in his 60s with no slack in his career-- I'd make him my personal mentor and figure out how he did it-- but it's quite rare to find such a person.
The reason ageism exists is that most people have to take low-quality, career-toxic work to (a) avoid income volatility that becomes more unacceptable with age, (b) evade the "job hopper" stigma, and (c) remain employable without moving to another city every 3 years, and this creates a world in which premature decline is far more common than it should be.