If think what you lads need to remember is:

a) The purpose of physical training is to first stress and damage the body in a way which stimulates and forces it to repair, grow and adapt so it cna cope with the stress you put it under. Your body cannot repair itself or grow as you train. It can only do so when you are at rest.

Lets say I took a brick off a wall (ie: training) and every time I did it a brickie put 2 bricks onto the wall (repair and growth) what would happen if I didn't give him a chance to put the bricks back on the wall? What would happen if he wasn't being supplied with bricks (food)? The wall would become smaller (ie: your body becomes less efficient)

training is only 50% of fitness development, recovery is the other half, if you don't allow for it you will either stop progressing or start to go backwards.

b) your still physically developing, F*** I'm 19 and I'm not fully grown yet but the training I can do, the periods (weeks... months... years) I can do it for, the intensity I can push myself too and the recovery that my body can make from it compared to when I was 16 are huge... when I'm about 23 I will likely look back at my fitness level now in teh same way I look back at being 16 now.