I didn't watch the video so can't precisely comment on that (I started watching it and, judgemental me, was kinda put off by how young she was coupled with her "perfect" showy makeup - if she was really that horsey, where's the dirt streak?
but that's just me being unreasonably judgemental).
But, if the other posters are an indication of what the "meat" of her subject was, I think it REALLY, really, depends on the horse whether or not they should be retired.
For instance, my 27 year old Lacey girl recently went from about 50% blind to about 85% blind (with most of that remaining sight in just one eye). She's in pain on a near daily basis due to that (small daily doses of bute help so she does get a bit of bute everyday) so I figured that now might be a good time to start easing her out of hard work (45+minute rides, averaging 5+miles/ride, three days a week is her usual schedule), right?
With just one long ride a week (she's also ridden twice a week, very lightly, by two young girls that I give lessons to) the life just seemed to start seeping out of her and I had no clue what was wrong. Then 2-3 weeks ago I started easing her back into her normal schedule again. I missed riding and figured I might as well try to cheer her up.
And what do you know? My happy, sassy, old lady is back with a venegance. She's trotting/cantering around her feild, building muscle like crazy, and does not want to come home when we reach the end of our intended trail rides.
Apparently, she wants to be the blind old lady that could.
I ride her three times a week now, she's ridden lightly in two lesson a week, and I lunge her walk/trot on the other days. She's loving it. Apparently, 27 and nearly blind isn't old enough to not be ridden.
On the other hand, at the summer camp I worked at for the last 5 summers, there were numerous healthy older horses that wanted to be DONE. Those guys ranged in age from 17-25 and all they really wanted to do was sit out in somebody's pasture, have a little girl put flowers in their manes, and just live out the rest of their days quietly and happily.
However, we HAD to use them. I felt terrible doing it but we had no choice. They were obviously unhappy undersaddle - cinchy with no cause, totally ignored their riders, spent more time eating on trail rides than actually walking, basically every behavior that says "Let me be done NOW".
It was the saddest but those horses were definitly too old, mentally, to be being ridden.
And then, there's the horses that Copperhead mentioned, that take 3 days to recover from a 15 minute ride. Or the ones that any "forced" exercise makes weight fall off of, those guys are the ones that need to be done being ridden/worked.
I really think that sometimes people need spend less time making statements about horses and more time truly listeing to what the horse is saying.
If you looks closely and carefully, mostttt horses WILL take the time to tell you about how they feel about one thing or another. You, as the rider/owner, just have to take the time to see and understand the signals you're being given.