I don't see why you'd normalize for that. I don't doubt that age is a factor, but surely the effect due to age should actually be counted. For example, different safety standards in cars will result in different fatality rates. That doesn't mean you try to eliminate that from the picture by normalizing for it, that means that the country with better safety standards is actually safer. If a country's roads are safer because they discourage or prevent younger drivers from driving, should not that additional safety count? Taken to an extreme: if a country allows 8-year-olds to drive cars and has a massive road fatality rate because of it, that shouldn't be normalized away when comparing with a country where you can't get a license until 18, right?