I am 23 (started working in programming at 22) and this view confuses me. I don't want to sound rude but... you didn't have hobbies? Friends? Partner? Errands to run? Non-work programming projects? Even when I was single and lived alone, I had plenty of reasons to leave after 8 hours.
Again, everyone lives life differently but I can't imagine having no reason to leave work, at any age.
It's not healthy in the long term but I can see why someone would choose that path and it doesn't have to mean this person "doesn't have a life".
Right now I am more resentful of the forty hours a week I spend at work than I was of the seventy I spent at work during the period I was practicing over twenty hours a week for a national dance championship.
When I was 23 I had a very active fun social life as you appear to have as well. BTW I had the same opinions as you at that age as well.
The difference is most of those activities are at your discretion and OPTIONAL and you have enormous amount of 'myself' time available . When you have kids and a family your OPTIONAL activities diminish and your 'myself' time is non existent, due to REQUIRED commitments.
In addition staying up to 3am or 4am or all night sometimes was common for me at that age.
After having a kid and family you can still do that. The issue is your kid doesn't give a F that you stayed up till 3am to fix something and you have to take care off all your REQUIRED responsibilities throughout the day. After a while it takes it toll on you.
BTW companies especially the big ones will push you as far as your demographics or industry will allow them too. Their priority is too push the "what is acceptable line" furthest as possible to save that dollar. The smart ones know better of the consequences, but most are too focused on the quick savings.
BTW when I stayed late or worked for free it was never for the benefit of the company. It was always for my benefit as I was never under the illusion that any company considered me more important than their bottom line. I viewed giving away my free time as an investment on my part to get experience in something that benefited me in the end.
You'll soon realize that side projects, friends and hobbies are nothing to be compared with having a spouse, responsibilities and taking care of a young child or an elderly.