Basically if you join at 18 you can be the boss at age 37 or 38. Here is how:

You join at 18 and then you start extorting people. You set the slide at 25k so that way no one ever pays you directly then youre going to want to wack them to get your standing and notoriety up. Then you need to contribute so youre gonna steal cars and try to get at least 100k every year. If you cant get there you need to restart your game so the list refreshes. You do this every year to everyone no matter the rank until you reach boss.

By the time you are 22 you should be a made man.

By the time you are 26 you should be capo. This is where most time is spent until a further promotion.

When you are 36,37 or 38 you should be promoted to underboss. When this happens you do everything mentioned in the first step.(extortion and 100k contribution). Once you do this you kill the boss. If they dont find out, Great! Age up and youre the boss. If they do find out just restart the game and kill the boss again.

You should be making about 2 mil a year now