I'm 25, and I definitely put in less than 100% at work. This is ONLY because I work at a very relaxed company and am a high performer, so I can get the same amount of work done in 35 hours as others would take 40. The trick is, I DONT take on extra work for those extra 5 hours.
My company is very informal and ok with working from home once or twice a week. So Friday afternoon rolls around and I have nothing left to do; I answer emails and chats as needed, but otherwise the afternoon is mine to go home early, do some errands, veg on the couch, etc. I check my email once at like 4pm and handle anything pressing that came up but otherwise I leave it alone.
I don't flaunt it. DH and I are still have student loans to pay off for the next 2 years so I don't want to get myself fired or something. If I worked for a more rigid, strict company, I totally wouldn't do this, but my company culture allows it so I take liberties where I can :) And thats not to say I dont provide value to my company, I totally do; I work the same amount as other people but I just happen to get a few extra stress free hours too.
This means my work-life balance is great. Being so relaxed means even when rarely emergencies come up, I approach them much more calmly now. I know that I have an "extra" 5 hours to handle things that come up if necessary. I've also noticed as I've become more confident, my peers/bosses are oddly trusting me more. I'm not entirely sure why this is, but after I've taken a mental step back, I must seem more capable or something lol