cupiscent
Sage
Depends on the point of the mentor. The mentor doesn't have to be able to coach you in everything about life necessarily. So if our Hero has been learning how to farm, but Miss Ninja over here has been trained from infancy to kill people with her bare hands, even if they are the same age - or she's younger - she can be his mentor in the art of killing people.
My next point would be: if the mentor character isn't around long enough, or important enough to warrant deep character development... why is that character there are all? Make it a book. Or better: make it important.
My next point would be: if the mentor character isn't around long enough, or important enough to warrant deep character development... why is that character there are all? Make it a book. Or better: make it important.