Old people will leave and new people will come..This circulation will keep on going and by 2020 I strongly believe the number will increase as compared to today but we cannot predict by how much it will increase. To be fair, the number can also decrease if bitcoin crashes by that time. It fully depends on bitcoin how it can attract new users.
Yeah thats true and if old people will be gone,we may see really big amount of people on whole world using bitcoin,
but i think that won't cross about 10%,remember there are a shitton of poor people who dont even own mobile phone