In theory, the big company would expect you to work 45 or so hours a week on average, and understand when you have to use your health plan and take time off when you're kids are sick. The startup, on the other hand, is giving you stock options and expects you to work 60 hours a week and oh, they can't afford for you to have (or use) health care, and you certainly can't have that much time off when the launch deadline is looming, but oooh, options, shiny!
All those other things cost the big company money, so the total compensation might be higher. The young don't care about those other things, and take a gamble for a big win with a startup. The natural cycle of life means older people can't do that sort of thing, so they gravitate towards other types of companies, those that know they can pay less in exchange for other benefits.
You can call it ageism or think it's fine but the fact is that ageless startups & corporations and their "big picture" masters regularly exploit the life cycle of their employees to maximize their work vs pay. That's how the business world works.