I think it should be optional voting 16-18. Only the politically aware would vote then. Can't really make it compulsory for under 18's because who would be fined for them not turning up and they're not adults so they can't really assume legal responsibility.
Personally, I've been pretty politically since age 14 or so and can't wait till I can vote. However, I think the politically passionate in my age group are probably a minority, but then again politically aware adults are probably the minority.
Btw, funnily enough I had a debate on keeping at eighteen and lost arguing that