The time between 20-35 is the most valuable time in your life imho. You will regret wasting this time for some death marches for a startup which promise you a lot of bullshit.
Of course you will regret it! Because, as I said, your time becomes more valuable as you get older. You're confusing good advice and empirical observation of reality.
Anyway, let me add to my previous comment. It cuts both ways. If someone hires somebody who doesn't think their time is valuable, they are fooling themselves too. Perhaps this person will spend 2 hours on ping-pong and 2 hours on Facebook out of their 12 hour shifts, because they do not value their time as much yet.
I disagree, but maybe that would explain it, your time is actually more valuable (for you) as you grow older.