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.)

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.