AZ: It does not disgard your arguement; they can marry the 19 year old girls who could care less. As you point out, supply and demand.
HOWEVER, if the older girls would have been willing to go out with these (type of) boys earlier, they may be married at this point. Not to blame them (they may prefer not being married), but the point that the “good” girls have fewer boys in the “pool” (or on the island) is also a good one.