At the age of 16, citizens lack the maturity to make such weighty decisions. They also have little stake in the country as they are entirely provided for by their parents, so issues such as taxes, etc., have little meaning to or effect on them. They are protected in many way (such as reduced criminal sentences before they reach adulthood for most crimes) because of a known lack of ability to make good decisions and to know how those decisions will effect the rest of their lives and the lives of those around them.
Lets say that 16 year old young adults suddenly gain an interest in politics (which is unlikely) after we allow them to vote, and have the knowledge required to vote effectively (i.E. An understanding of politics, a knowledge of the candidates/slash laws being presented, etc.). What if, all of a sudden, we think, 'Well, what about 14 year old kids? They seem to be smart enough and have the capacity to vote. Lets allow the m to vote as well!" So we give them the right to vote. Then we might think, "Well, what about 12 year old kids? My twelve year old seems to be smart enough to vote!" So we give them the vote. Eventually, we might end up with eight year old children voting! We cannot allow 16 year old young adults to vote because it would be the beginning of a slippery slope. The question we must ask ourselves is, "Where does it stop?"