We got guys who go full time right when they turn 21 now. Start at 18 and go full time at 21. Their career isn't the biggest impediment to a happy life.
The most important ingredient to a happy life is who you allow to be in your life. This starts with who you date and this is the biggest problem I've seen with the guys who go driver at 21. They date foolish. They get advice from the sad sack divorced guys. Pro tip, don't take advice from guys who have failed once and never won again.
After who you date which friends you allow into your life will be the next most important part of your life happiness. Choose your friends wisely. Don't hang around bucket crabs they will pull you down so they don't have to accept that you can pull your way out of the bucket.
UPS is a good career for the right kind of self motivated person who can look at the glass and say it's half full. Who can see more work every day and think, heck yeah more work means more money. Planning at your age? Pretty smart move. Make sure you take care of the first two things I said and you'll be good.
Family > friends > career
You're really analyzing your young co-workers dating lives? That's pretty

ing weird.
21 year old kids should

as many women as possible, without knocking them up, and have fun. They should set aside some money and start planning for retirement, yes. But they should also waste some money and have fun. You're only in your early 20s once.
Finding the right woman to settle down with should be the last thing on their mind - it will just happen, especially if you sample many of them. The right one will be a standout, and not in the sack, and not in the looks department. A standout person/personality.
The friends thing is good advice though. You will have "friends" who are on the wrong path when you are young - identify them, and drop them. They will bring you down if you give them the chance, even if it's not their intention to do so. If you're working FT at UPS by 21, you have life by the balls. Keep your grip on them

s tight, and enjoy the ride - you'll be a wealthy man by the time you retire if you take the right path and start planning at that age. You will be amazed at the magic of compounding returns over a 40 year period.
Good luck, kid. Have fun - YOLO MAN!