How old is too old to be a world leader?

When it comes to being a world leader, how old is too old? Voters in Malaysia will soon have to ponder that question, after the country's opposition alliance selected former prime minister Mahathir Mohamad to lead its challenge against Najib Razak, the scandal-ridden current prime minister.

At 92, Mahathir is almost three decades older than Najib. He…