You can become a Mentor from level 30.
Any new character can receive a Mentor from level 9, and they can either choose to stop being an apprentice, or they will stop being your apprentice once they have reached the maximum level. Apprentices/Mentors are chosen at random from people who are online at that moment. Unfortunately there are no exp bonuses for partying with your mentor.
The benefits for the apprentice: If you find a good mentor, you can receive advice on your class and help with builds or in general about Allods. Also, by doing the Apprentice quests with your mentor, you can receive a nice Epic piece of gear every few levels.
The benefits for the mentor: You receive a % of the Anniversary coins the apprentice earns, increasing as their level increases as well as Marks of Diligence if they are in a guild. For doing the apprentice quests with them, you will receive some gold, increasing with their level also. You may also have that nice feeling that you are helping a new player through the game
Downsides: It's random so you could get anyone. They could be good, or bad, or not even speak your language. You may get someone who quits the game. A lot of new apprentices quit quite early on :/ Other people may find their apprentices are annoying, begging for gold, etc etc. However, it's always possible to reject an Apprentice/Mentor if you don't like them