Be sure of what you want
While your age should not be a barrier and you certainly have an awareness of the key operating platforms, you should be warned that teaching is a skill in itself and that it is a very demanding profession. You will need to consider seriously if you have the skills, patience and the tolerance to teach people with very little knowledge of your subject.
Without specific teacher training it is not going to be as easy, as the competition is very high. I would advise that you spend the next year studying to become a professional trainer and gaining the skills and experience that will enable you to train both experienced and inexperienced students.
At the same time you should consider approaching local IT training companies to see if you can gain some work experience with them. You could offer your services at a reduced rate until you are up to speed on the product set. This way you could test the water to see if teaching is really the profession for you.
Once trained, I would advise that you maintain and build on your knowledge of Windows 2000. As more and more organisations migrate from older operating software, there will be a real need to train office staff in the latest Windows platforms. Do not just limit yourself to private organisations either, there are some very large government sites such as the Department of Social Security that are going through their migration process. One tip would be to try and ascertain who does their training. It may be worth approaching them or their consultancies directly.