R.B.S., as long as you can maintain a class 2 medical you can continue flying up to any age if on aerial work which is what PPL instructing is. You should have no trouble getting part time work, maybe you should look at FIC qualifications as well, make yourself even more valuable! You will have no trouble with the instructors course from your A2 background, you will also be exempt from a formal course, you will just have to do suitable training to pass the test, good luck!