How quickly can you send oxygen to your muscles?
The results are pretty straightforward. The trained group is always higher than the untrained group, and both groups decline at a similar rate of 8-9% per decade as they age. There's been some debate over the years about how much of the decline in VO2max as you age can be attributed to aging itself, and how much results from the typical pattern that we get less active as we age. In this particular data set, it looks like the decline is pretty much entirely attributable to age, because the cyclists lose VO2max just as quickly as the untrained group. (Of course, they start from a higher point, so the 66-year-old cyclists are just as aerobically fit as the 24-year-old non-cyclists, which is pretty cool.)
What this study is actually focused on is something slightly different: VO2 kinetics – that is, how your body's use of oxygen responds to changes in activity levels. When you go from rest to suddenly pedaling or running hard (as you do at the start of every race or even interval workout), your muscles suddenly need huge amounts of oxygen, and it takes a while for the rest of the body – lungs, heart, blood vessels, enzymes – to kick into high gear.
What does this mean? The factors affecting VO2 kinetics are fairly complex, but one key is the blood delivery network. In order to quickly ramp up oxygen delivery, you need to be able to distribute blood; studies have shown that less than a year of endurance training increases capillarization by 20-40%. This could be one of the key factors that you lose with age if you don't stay fit. It's probably not going to be the deciding reason that you take up training ("I've decided to get fit because I don't want my oxygen kinetics to decline!"), but it's a nice example of the many subtle ways that your body keeps working if you keep it fit – and stops if you don't.