Rights

"The rights based argument maintains that as voting is the central way in which citizens express their judgement and support of government policy, it is only fair that those who are affected by major government decisions are given the opportunity to express their opinions via the ballot box. The most common examples of these are the responsibilities of joining the armed forces, raising a family and paying tax, the argument being that if you can die for your country, get married and pay tax, you should have the right to indicate your feelings to the government." - Electoral Reform Society

It is a good argument. However, statistically, most 16 year olds are not in the army and they are not married and/or raising a family. Most of them are also uninformed and generally ambivalent toward the politics.  
 
Yes, on the face of it, if 16 year olds can "die for their country" and "get married and pay tax", they should also be allowed to vote. Yet, how informed are their decisions in dying for their country? In getting married and raising a family? Not very. So it would be with the lowering of the voting age.