If you are Coaching a Junior or Youth team you should be using the 1-4-3-3 system of play.
If you are coaching according to the FFA National Curriculum......
Younger teams play the midfiled triangle "Point Forward". Put simply, one player is at the point of the triangle is forward and behind the front three players, while the two other players are to the rear but in fromt of the back four players. Essentially, the emphasis is more on defence than attack.
However, at about 14/15 years the formation in the midfiled is changed to "Point Back". The midfield now plays a triangle that has the point in front of the back four, while the base of the triangle is now forward. With two players in the midfield triangle behind the front three, the empahsis is on attack.
Its really important that the young players get a chance to play this position and for them all to know the basic repsonsibilities of each position in the midield. Well all positions really. But give them a chance to play these midfield positions.
When you play point back, the single player at the base of the triangle has a lot of responsibility. He / she is now called a defencive midfielder.
The video below is very instructive on the subject of the Defencive midfielder.