Let's assume you know a lot in your field and have acquired all the required skills over multiple years of experience. You begin to mentor someone who is already bright and inquisitive to learn everything fast.
By seeing the rigour of this mentee, you feel threatened and you begin to think that the mentee will become better than you if you teach everything you know, so you start to restrict the mentee's learning.
The bright mentee feels under utilised and move out of the job. You continue to rule your team with a bunch of averages and continue to do your job in the most regular fashion.
End of story.
The moment you think that your existence is going to be challenged if you let out everything you know and you begin to withhold your knowledge within yourself, then the only person you should have felt threatened about is not your mentee, it should be you.
Learning the skills you know doesn't make anyone a better person than you are. Knowing when to use which skill and the way to use that skill is what makes you YOU, and that comes only with experience.
Even if this mentee is capable of becoming a better version of you in a much lesser time than you took, why should there be a space for jealousy.
When you have a stronger version of you beside you, you and your team are more likely to achieve a lot more than what you usually do. It drives you to become a better version than you are. Isn't that something you should nurture more?
A wise man once said, "if you are the smartest person in the room, you are in the wrong room".
The choice is yours to make.
But the choice you make here will determine whether you want to "just live and leave" or "live and leave with a legacy".
Good luck!