By total coincidence I’m just reading a book where this question is asked and answered. The answer is 32%.
I read that book. It asks you to guess before answering, and mine was about 20%. I am usually much better than that at making wild ass estimates so wondered why I low-balled.
Thinking harder, the world has 8 billion people but many will never travel in a plane. The US has under 5% of the population. Flying in a plane takes money and technology. If the US has 330 million people, this is a lot more than most other G7 or G20 countries. The other factor is open space - plane travel is important in Canada and the US but in Europe it is often fairly easy (in principle) to drive or take the train. So I thought the factors favouring airports are population, size, wealth and technological prowess.
So if one excludes the heavy populations of India, China, Indonesia, Pakistan, etc. and takes the population ratio of the US to other wealthy countries, it comes out close to 33%.
However, this number includes a lot of private airports and cargo hubs. There are many ways to define an airport. International ones that can allow people into a country are much less numerous, for example.