It depends on what is an "old game" too. If Counter Strike 1.6, NFS: Underground 2 and World of Warcraft are "old games"", then I don't know any teen who DOESN'T play old games. If you mean DOS games only, the number will be much smaller.
As a side note, some people on the internet stated that there is less than 10000 people in the world who still play old games. I think that it is nonsense as I have 4 friends who play DOS games and I'm from a 25000 people town, but what's your opinion.
In my opinion, more people play Windows 9x/DOS games than CoD or Crysis actually, because the vast majority of world is not "first world". My uncle only had a Pentium 166 MMX until a few years ago (through he is not a gamer, he used it for work and web browsing) and I think that in many less developed countries most computers are still old DOS machines. I have a friend who is 17 now and he only got his first pc 5 years ago, a secondhand 386SX, 16 Mhz (followed by a secondhand MMX Pentium 233, and now he has a new laptop, although it still has a crappy integrated card)!
Really, sometimes it seems that some people think the whole world is the first world where any stupid kid can whine and gets an Xbox 360. I know plenty of people my age who didn't even have a computer until they were 10 or so, I was a lucky one. And Slovakia is comparatively awesome in this regard, I've read that many countries in Latin America and Asia still have Famiclones being sold, not for some stupid pretentious "I am a hyper retro hipser", but for average people in those countries for whom a PS3 costs the same as food for a family of 4 for a month.