Robert Bruce was 21 when he first learned to astral project consciously (had projections before this age as well). Robert Peterson (Out of Body Experiences: How to Have Them and What To Expect) learned to project at the age of 21. Anne Varnes (Astral Voyage - Astral Projection and Spirituality
) began astral projecting in her teens I believe. Also, John Magnus (Astral Projection and the Nature of Reality) learned to project in his early twenties. I could be wrong but older projectors like Robert Monroe for example were more the exception than the rule it seems.