That is not that young.
Tell them to date for a year and see if they feel the same.
I second the ‘1 year’ idea. When young people are sharp and really know themselves for their ages, I think young marriage is a very good idea. But they do need to go through some time together, with marriage in mind; some time to experience a few difficulties, and to see if everything ‘sticks’ through the difficulty.
Best wishes to them!
-JT