A lot of this analysis varies dramatically based on location. I say this in terms of both sides of the equation, one being income opportunity and the other being cost of housing/living.
Interest rates for mortgages are an important factor, but for the purposes of evaluating pure affordability, it is not hard to look at the ratio over time between income and housing prices. This is basically what Doug has pointed out above. Over time, that ratio has spread significantly, meaning housing is much less affordable in relation to income today versus in the past. Personally I think those of us that got in when we did should be a little bit grateful, and the younger generation that is trying to get a foothold in today's economics deserve a little bit of empathy. Consider education and medical cost, and the picture becomes even worse. Maybe not much of a concern for those of us now on Medicare and/or who obtained their education when it cost 1/10 of what it costs today.
Again, this can vary a lot from location to location. And there are always plenty of anecdotal specific examples or outliers that will make any case you like, but don't let those distract you from a larger trend.
What I am referencing above is more of an average scenario in urban areas nationwide.