The Mentor skill allows you to mentor anyone to level 4 or 5 of the skill you are maxed out on. Not all skills can be mentor by maxing out the skill. The skills it affects is the music instrument skill and the fitness skill. The mentor trait unlocks the other skills to be mentored such as cooking, dancing gardening and writing by talking about said skills. Skills that you are 4 levels above the other sims you can mentor them with that reward.
The mentor trait does not affect the physical and music skills and has no impact on though skills you need to reach unlock which is 8, 9 or 10 depending on when it unlocks in that skill you will get a ask to mentor and ask to train. I know its a little confusing.