Need an argument against lowering the voting age?

Try this:

According to the Kids General Election survey, 15 percent of 9- to 14-year-olds surveyed by the Cartoon Network think 19-year-old Manchester United and England striker Wayne Rooney is the best man to be Prime Minister. Fictional wizard Harry Potter also garnered a greater share of this yoof vote than Tony Blair.

But seriously:

Instead of Iraq, immigration and the economy, youngsters would want a Prime Minister Rooney to address bullying, school dinners and the burning issue of the recent break-up of boy band Busted.

(Speaking of arbitary age limits, I like the idea of U.S.
states lowering the drinking age back to 18.)