It's true, most good players are actually a little older, like mid 20s.
But the key things are.
1) Learn the maps. You should always be either in the perfect spot or going to the perfect spot. If you are ever just wandering then you are not playing right.
2) User your grenades. If you are dying and you still have both tactical and lethal grenades you are not playing right. If you are approaching a high traffic area, throw a grenade in there. If you are approaching a building or corner, throw your flashbang. etc.
3) When starting out, work the outside of the map. I'm still shocked by how many people I see running across the middle of the map. You want to be places where the number of sides you can be shot from is the lowest. In the middle of the map you can be shot from 360 degrees..lol.
4) Don't stop moving while shooting. Shoot and move at the same time. This part is a bit more advanced but move away from the direction the other guy is shooting. For example if a guy is coming around a corner, it will be easier for him to aim towards the corner than away from it, so you should start shooting and strafe away from the direction he is turning. With some practice this becomes automatic and you'll be able to beat most players when working your way across a map.
edit: And don't sprint around corners.
