There have been many cellists throughout the years that have joined the same orchestras that Rose was a part of. Some musicians have joined these orchestras at a younger age than 26 and 21. Therefore, the age that Rose joined these orchestras does not prove his ability as the greatest cellist of all time.
Additionally, it is not uncommon for a musician to become a teacher, especially after their experience in an orchestra. As a talented musician, it is no surprise that Rose could teach cellists his skills. This means that Rose is not any more talented than other cellists that have played in orchestras and then go on to teaching at music academies later in their life.