You should always do things that play to your strengths. You should shy away from things that accent your weaknesses. You should have an advantage in what you do.
One of the core advantages that a 20-year-old has is sheer energy and less outside responsibilities. A 40-year-old has a lot less energy and has many things other than work that take time (like kids, spouse, elderly parents, community service, boards, managing the money they made, and more).
Young people have fewer constraints on their time and thus have the ability to work much harder.
Some people work hard all their life ... but most people work their peak weekly hours at about 25. It is extremely rare for a 55-year-old to work more weekly hours then that same person did 30 years before. (of course, there are some exceptions)
So if there is ever a time to put in raw hours, it is in your 20s. You'll likely benefit immensely from the time you put in because you will distinguish yourself from your peers, grow faster, and earn more.
If you work more you'll also spend a lot less time watching TV, playing video games, and on social media (which is not very healthy).
The downside of working long hours
The downside of working long hours isn't what people normally write about (lack of social life, etc.). The main downside is not giving your mind time to wander and explore new things.
A good way to encourage exploration is to schedule time to explore. When I was younger, I would block off 6 hours almost every Sunday to read random things, think about ideas, etc.
Part of exploration might also be taking time to travel, go to museums, and try new experiences. All of which are more difficult to do when you have core time constraint responsibilities when you get older.
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