The results are generally valid ... there's both an internal & external consistency to the inventory. It doesn't change much over a lifespan. As someone said, Kristi I think, there can be changes within a person's range, but as she also said that tends to be from life-events, experience, not from errors in the inventory.
In general, if someone answers the inventory truthfully every time they take an inventory the letters should remain the same over a lifespan, although the scores that form those letter designations will, and do, change.
Nichole