Thanks for the comments. You are right that the line plot is not a plot as time goes on. It is a snapshot of people of different ages at one instant in time (2007). The distribution will change each year, and by the time you're ready for your hip replacement, the distribution will look quite different.

Your comment about the discrete vs. continuous nature of the horizontal axis is very relevant. I chose to make the horizontal axis look continuous because the categories, although discrete, are a binning of a continuous variable. I was thinking that if you want to estimate the percentages for, say, a 26-year-old, you could look halfway between the third and fourth tick mark.

However, since the bins are different widths (the 18-21 bin encompasses four years whereas the 27-40 bin encompasses 14 years), using a continuous axis causes distortion. This was probably not a great choice that I made.