It is very easy to argue against this point of view because it makes very little sense at all. There is no reason to argue that the fact that not many people in this age group vote means that none of them should be allowed to do so.
First of all, America has been dedicated to expanding the franchise, not contracting it. We do not want to become less democratic just because younger people do not vote as much as we would like.
Second, it would be unfair to prevent people in this age group from voting. We recognize these people as adults in most ways. Most importantly, we allow them to risk their lives in the military. If we allow them to kill and to die for us, how can we argue that they are not worthy of voting?
Finally, taking away their right to vote would be like collective punishment for the misdeeds of a few. If one young man from a given neighborhood commits a crime, we do not imprison him and all those who are in the same age range. That would be unfair.
Thus, denying 18-21 year olds the right to vote for this reason would be un-American and unfair.