I'm probably old by Valley standards (32), but I took a lot of crap for this view in my twenties. I worked for a consultancy that made money by billing my time out. They'd never tell me I had to work more, but it was heavily implied on multiple occasions that working beyond 40 hours was preferable. I asked why, as I had a series of successful deliveries, and stayed ahead of my workload. They would always terminate the conversation there.

You have to play close attention to the incentive structure of the business. The situation is arguably worse for startups if you're salaried: you're 'changing the world' so it becomes OK for the occasional 50/60 hour week.