The 25-year old knows that if his startup fails he can easily find a first (or new) job at Google etc two years later. That does not apply to 45+ year olds. The risk is much higher.
I don't get this, presumably by that point in their career they'd have a huge network of people they've worked with in the past and who would be eager to hire them ?
When you're at that point in your career you cost a lot more to hire and there are fewer positions, especially since it's usually better to promote an insider instead of hiring a buddy from outside the company.