Does this mean that the average buyers are the same people for 15 years?
Those who are 56 now, were 42 in 2010.
If that's true then it means the statistics include people that sell their home to buy a new one. Not sure what lesson we should draw from it but perhaps it's not just that young people don't buy homes anymore (if that's what you wanted to observe then find statistics that show the age of first home buyers)
So it looks as if the cohort buying has stayed roughly the same over the years. The average buyer was born in 1970. Bad enough. But when we consider the bell curve, to compensate for the older tail getting decimated by age it must have gotten even worse for younger cohorts than what "the average rises by one year per year" suggests at first glance.