This being said, it's unusual for a democratic head of government to be anything other than a grey-hair. Other movers and shakers, sure, but to climb to the very top in a democracy generally takes a while.
I don't think this is true in Western democracies, where age can be a liability during an election. Advisers yes, but the guy running on the ticket needs to project charisma and vigor as well as some wisdom. The funny thing about elections: the electorate doesn't look at your CV so much; Obama was just a 1 term senator before he became president. Having more political experience would have actually been a liability for him as he would be seen as more of an insider.
This is definitely true in the directly elected presidential system we have in the states (definitely true post Clinton, even Reagan was an exception). Parliamentary systems are a bit different since the prime minister is elected indirectly, but even there we are seeing some fairly young heads of states in the UK and Germany (Cameron is 40 something, Merkell has been their 7 years and hasn't even hit 60 yet).
Perhaps New Zealand's an outlier, but the majority of Prime Ministers here (functionally the head of government) during my lifetime came to the office in their 40s. I note Cameron and Blair in the UK were in the same age bracket.
