Youngsters progress

I thought the 3 point line was beyond the free throw line. So if they work on their 3 points shots they are working on their shot from beyond the free throw line. Why stop at 16 - 18' and not go all the way. Remember when you were in high school and ......

Which 3-point line are we talking about. That diagram makes it look like there are two of them, one on each end of the court.

That is good question.

I think both ends of the court.

Otherwise they will only be effective from beyond the 3 point arc for half of the game.

Not all ends of the court are created the same.

Rim at one end might be tighter
Air conditioning/heating breeze may be stronger at one end
One end my have a dead spot or two in the floor that disrupts the players rhythm and negatively affects their shot.
When shooting from beyond the arc in front of their bench they may feel self conscious
Etc