Well, if young people are allowed to join the armed forces to fight for their country, set up businesses and pay income tax, get married and have a sexual relationship, change their names by deed poll, pilot a glider or drive a moped then Shirley they should be capable of casting a vote in a democratic election.
Younger people lack in political knowledge and are dependent on their parents, I feel that lowering the age will just support the parents views because its likely for the younger person to follow in the same footsteps as their parents and vote for the same candidate. THEY ARE TOO YOUNG
Younger people usually lack the political and economical knowledge when compared to older people. They are also more susceptible to following trends. They don't usually have to take care of their own families nor do they have much working experience (if any at all), all of those very important qualities shared by the people who get affected by political decisions.