1) History has taught us, there is nothing to be learned from history, especially from those who lived it.
Companies (especially Startups ) can only flourish if they repeat the mistakes of the past over and over.
2) Older workers cannot fit in culturally.
Living say into ones 40's (God forbid 50's) does not equip these workers for team collaboration or contribution. Studies show, as people age they lose all social graces and empathy for all around them and could not possibly understand the complexities and social dynamics as well as say, recently or near recently pressed college graduates. Besides, younger workers might feel "funny" with them on the team (not to mention cooties).
3) It's always better to have gifted and talented newbie's led by other gifted and talented newbie's.
Everybody knows, leadership is in-born not learned and honed over time.
4) It's better to have lots of gifted and talented lower paid newbie's doing a single job or tasks over and over again without tenured guidance (I believe Wharton calls it "Trial & Error" style of management) .
Employing an old bag who might help get it right the first time is a waste of money. Besides, the office looks more busy when everyone runs around aimlessly. Investing in "free bagel Fridays" and unlimited coffee and soda (sugar and caffeine always improves the dynamic) is a far better "people" investment.
5) There is no moral or ethical obligation to hire them.
Studies show the vast majority of those reading this will be comfortably retired with HUGE retirement accounts by their mid 30's so what's the problem?
6) Hiring Diversity is a good thing!
But... The Age Discrimination in Employment Act of 1967 (US) was never amended to include old bags so why bother. Compliance is inherent because "AGE" does not discriminate on the bases of Disability, Genetics, National Origin, Race/Color, Religion, Sex, Veteran status so can we REALLY call it discrimination?
I could go on, but you get the picture. If ever in doubt, return to rule #1 and you're company's success is almost surely assured!