Beowulf,
I would recommend reading this page, which discusses the different types of variables:
http://www.sciencebuddies.com/science-f ... bles.shtml
"The independent variable is the one that is changed by the scientist." This does not mean that you change whether or not someone is a smoker. It just means that you manipulate that variable by grouping people into smokers and nonsmokers.
"The scientist focuses his or her observations on the dependent variable to see how it responds to the change made to the independent variable." The length of time that people held their breath is the dependent variable, because you tested how this value differed depending on whether a person smokes or not.
The terms "dependent" and 'independent" do not relate to whether a person can change one of their physical attributes. Even if a person can not change their height, that could be a dependent variable in a study of how, for example, nutrition affects a persons height. The person cannot control their own height, but the scientist tries to control all other variables (gender, smoking, race, etc) and to look at people with different nutrition and different height to determine how nutrition can change a persons height, all other things being equal.
I'm not sure what you mean by "the study could have been different with different test subjects". Ideally, your experiment would give the same general results even if you chose a different group of study subjects. If your results change when you rerun the experiment, it might indicate that there is a problem such as an insufficient number of test subjects in the first experiment. Conversely, although a person can change their weight, their weight would be an independent variable in a study of how weight affects occurrence of heart disease.
I don't know your results well enough to know exactly what you should write in your conclusions. If you did not see a difference between smokers and non-smokers, you could say that the results did not support your hypothesis (if your hypothesis was that smokers would not hold their breath as long). You could add that this may be due to factors such as age, fitness, etc, that may also affect ones ability to hold ones breath, and that additional experiments that control those variables might give different results. You may or may not want to discuss variables as being "uncontrolled" because it sounds as though your teacher has not introduced this idea. You should ask your teacher. As noted above, the number of subjects could also be a factor. In scientific studies, hundreds or thousands of subjects may be required to test these kinds of hypothesis where there are many influential factors such as weight, height, etc.
As a side-note, these terms "independent" and "dependent" come from the idea that the independent variable is not affected by the dependent variable, and that the dependent variable is affected by (or depends on) the independent variable. For example, it makes intuitive sense that if you smoke, your lung capacity will be somewhat reduced, but it is unlikely that ones lung capacity would affect ones decision to smoke or not. However, in many studies, the relationships can be much more complicated, and the idea of one variable actually being "independent" isn't as clear. In any study, it is worth asking the question of whether your "dependent" variable could actually be affecting your "independent" variable. You have already done a good job of considering what other variables might influence results.
I hope that helps.
Chris