I think you looked at correct data but got to the wrong conclusion.

The software industry is relatively young. How many people started as programmers 30 years ago? much much less than 10 years ago or last year. In the mid-90s, the software boom started and the number of youngsters that became programmers (like myself) increased drastically. So while it might be true that some programmers get promoted or retire, the reason that there is such a small percentage of older programmers is simply, ummm, that there are so many young programmers coming every year.