go to the supposed to be pupil interaction menu, if his ready, there will be a number of instruction to choose from, saying that you think this 18year old lad will benefit from some other guy or so...on the lowest should be a drop down menu so you can choose among the few that can be the mentor/tutor...
please note that normally only those playing in the same positioning will be paired... also depends on the player, he might like your choice for his tutor or not... ie, a 18yo DMC/ML probably prefer ur 31yo ML as his tutor than on form 29yo DMC... although u might thing he should be ur next DMC

lastly, not every young players will have this choice on their interaction menu... if they have one before, n they say they don't like ur choice, the choice will disappear from their interaction menu for a while so u might have to wait a little bit..
hope this help.