There will come a point in the relationship when the mentee gets to be more informed/experienced on certain topics than the mentor.
Even though the mentor might be older and more seasoned than the mentee, the mentee brings a wealth of life experiences to the relationship.
Especially because they (the mentee) have been so close to the mentor for so long, they might be able to see the blind spots of the mentor clearly.
When this point arrives, it is my recommendation that, instead of breaking off a relationship that took a long time/energy to develop, both mentor and mentee talk about the future of the relationship and the possibility of a collaboration/friendship. Otherwise the relationship would be wasted.
The change in the power dynamic may be difficult for both mentor and mentee, but a good source of growth (or, just like someone I know used to say: your pain is the compost- many wonderful things will grow out of that).
Mutual respect, a true desire to grow and a burning need for connection are essential for the new relationship to develop.