Why shouldn't you include people who spend nothing on music?
If you don't, you are skewing the statistics. Most people spend nothing or very little on music. Even five albums a year is unusually high. Teenagers and young people spend far more because they have plenty of free time in which to listen to music. Then people's lives get in the way, so they stop listening to music and buying it.
Enjoy your music and be happy with it. Just admit that you're not representative of the people.
Including the people that spend nothing on music, is skewing it. You tried using 7 billion as a figure earlier, that would include people that cannot buy music, babies, children, homeless, people that have no money.
Also, you're making a ridiculous assumption that adults 'stop buying music as life gets in the way'.
Also, if you're going to use the iTunes account figure, that includes duplicate accounts, accounts for children, accounts for people that have multiple countries (People in Canada have UK & US accounts, Some in the UK have US & Canadian accounts). All of that skews figures to be inaccurate.