Speaking as a former hairstylist, there is no way to increase the individual hair width to make it thicker or to make more hairs on your scalp. However constant abuse will make your hair seem thinner because it is breaking off, so you have the same amount of hair on your scalp but less hair as those hairs grow longer (again because some of them are breaking off). Plus when hair breaks off like that, the result can be more split ends which causes knots and tangles which, of course, leads to more breakage when trying to remove them. Once you treat your hair nicely, it will thicken up.
Hair does go through changes as we age and some of those changes can be less density. Your hair may become more dry as you age too, which can lead to more breakage and therefore a double whammy on the thickness of the hair.
If one is straight haired person you more than likely loose hairs throughout the day and they just slide out. On curly hair, they get caught up in the other hairs and really need to be removed gently like you are doing now.
Some hair loss from the hair follicle is normal, but what you don't want to see is shorter strands of hair all over your sink or shower, which is an indication that you are breaking hair off while you are de-tangling or styling it.
Also curly hair can seem thicker than straight hair because it takes up more horizontal space than straight hair. I hope that makes sense--so if your hair is 12 inches long when straight but only 8 inches when curly--your hair will seem thicker as the curls or s-wave takes up more space horizontally.