Evidence is that student double voting is more myth than reality. That said yes the loophole should be closed I suggest your voting age of 21 would seem more to be abput the tendency for younger voters to vote Labour than any real electoral concern. And anyway many students are over 21.
All that's needed is to make it illegal for students to be on two electoral rolls (which are easily cross checkable). Postal voting makes it unnecessary for them to have two options on the day.
I agree with that first paragraph Steve, it is also a fact that immigrants are also more likely to vote Labour, so clearly any attempts made to change the voting age or demographic are clearly attempts at interfering with election outcomes. This was also rather cynically used by the SNP in there indyref, personally I believe this should be legislated against.