21 is the perfect age to start, but the job isn't for you if you want to have a quality family life. That's the bottom line. Even those of us that are fortunate to work very little OT still don't see much of our families.
Here's an example of my interactions with my kids during the week during the school year:
6:30 AM: time to get up!
7:10 AM: have a good day!
8:30 PM: how was your day?
8:40 PM: have a good night, cya tomorrow.
And here's an example of my interactions with my family during the holidays:
Sunday after Thanksgiving: see you in a few weeks!
Christmas Day: hey, how you guys been?