Don't see why not. Mbappe is only a year and a half older is reportedly only available for near enough world record fees. If Madrid believe he can be a world beater I don't see why thry shouldn't be allowed to take a plunge on him even if he hasn't turned pro yet. It's a risk they're willing to take if he flops and one they will be happy they took if he succeeds.
If your issue with it is some dodgy way of paying for it then fair enough but assuming a fair payoff with no dodgy 3rd party ownership issues robbing the selling club, and if the player is happy to move then it's pretty fair for everyone involved. A huge windfall for a club that probably won't see that kind of money for a long time, a large hike in salary for the player (if he accepts and believes he has a chance of making it), and a potential star for Madrid.
