I'm only realizing now that someone my gym instructors are half my age. (The ones that are actually older are very impressive I must say.) Keeping up with the younger instructors is a challenge and is pretty empowering. Then every so often I am reminded what a disadvantage that I am at and this is why they put athletes in different age groups.
The disadvantage that older athletes face is the Maximum Heart Rate (I usually refuse to call myself older, but in this case, it's pretty inescapable). As you get older, the tissue stiffens and the maximum rate that the heart can go decreases. The bummer about this is that a younger person's heart can pump a significant more blood than an older person's and man I'm jealous - sometimes I wish I'd appreciated it then.
I overheard one of my gym instructors say that her near max heart rate was 195. Mine is about 174 or 175. The standard formula (220-your age) would put it at 173, but fit people usually can go slightly higher. This means that on demand her heart is able to move around that much more oxygen than mine. Because she exercises just about every day her resting heart rate is 38 (I asked one day) and mine remains at 58 despite exercising four times a week plus dog walks. That means that her heart is that much more efficient as well.
Because I can't get my heart to beat as fast as hers, efficiency and being comfortable at higher heart rates is the only way I can keep up. The standard text books are fond of telling you that you want to work out at 60-80% of your max HR. If you're no longer 20-something and training for anaerobic things like mountain climbing (or just trying to keep up with the 20-somethings), then you have to get comfortable working much harder. you get to learn all about how to work at that "anaerobic threshold" where your body is no longer processing oxygen. It's that leg burn phase and you can push that threshold up further.
My anaerobic threshold used to be around 160 bpm. Now it's 163-165 bpm. Yes it hurts - but only briefly. ;) The way through that is usually by doing intervals where you vary the intensity of what you're doing, while there are a whole bunch of different ones, right now I just use the ones in the RPM Spin classes I go to.
If you'd like a decent book on this subject there are 100's (mostly all pointing in different directions), but one good, albeit humbling, one (I'm lucky to match their fitness minimums and I often don't) is:
The Outdoor Athlete
by Courtenay and Doug Schurman of Body Results
So it turns out there is an honorable reason to (ahem) chase 20 year olds.