25% of the Earth's population is younger than 15. Assuming 20% of them use the internet, the TAM (not including the <15 year olds) is 80% of the world's population.
3 out of 4, or 75%, seems like nearly ever human, I'd say (though it is a matter of opinion I guess.)
Much like the cliché "the internet is the sum of all human knowledge".
If you have deep specialist knowledge in a field that has existed for more than thirty years it can become quite obvious that the internet really in is nowhere near "the sum" of our knowledge.
If we exclude over 75 with no interest in using it (and no use case for it in their environmnent/culture), and under 10, we're pretty close to covering all the rest.
And even the 60% is some hand wavy estimate about internet reach, about people using the internet frequently. More people use the internet than that, some transparently through some smartphone app or basic feature phone, which almost everybody has even in the poorest places in Africa.
Lol at you missing the qualifier "and no use case for it in their environmnent/culture".
Everybody knows avid users at 75+ or 10-, you haven't discovered something knew. But there are whole countries/areas/cultures where e.g. the majority of 75+ have no internet use, and no interest in getting any. You know, in the world, not in the US.
3 out of 4, or 75%, seems like nearly ever human, I'd say (though it is a matter of opinion I guess.)