Hello OP I am new to the forum but in the reverse, so-to-speak. I will be 70 this year and have been on family horses since I was two. Paying for my own, non-stop thru life since I was 12. I am down to my last two horses, ages 21 & 22; when they are laid to rest, below the barn with those who have preceded them, it will be the end of a grand Lifetime of horse ownership.
While 22 is no longer considered elderly, it is Senior, therefore you ask some legitimate health questions. Even with a lifetime of owning the same horse(s), it can be difficult to figure out when they are uncomfortable about performing a task and when they are just trying to get out of something
To assure my horses (which are lifelong-swim-the-River-and thru-the-woods-trail-horses), are in the best health they can be, given their ages and injuries, I have an equine chiropractor assess them. She is also a DVM and does acupuncture.
Since one horse is seriously insulin resistant and has foundered, she assess them frequently --- big owie on my checkbook but I owe him that for hauling my butt anywhere there wasn't a trail to follow
I am also able to watch them in the pasture. Nothing like "at liberty" movement to see if a horse is moving sound or having problems somewhere
How long since the mare has had a good physical, including eyes, teeth, some blood work, and a generalized "going over" of the body by a vet to look for soreness or stiffness?
Sometimes, not always, but sometimes standing and staring into the distance can mean eye issues. One of my horses has always stared into the distance and there is nothing wrong with him, lol. The horse with IR will stare and often shake his head --- the vet examined his eyes and declared he has "floaters" in his eyes like many humans get as we age. I empathize with the horse as I have lived with floaters fifteen years or more.
I hope you keep riding and enjoy many more years with this horse
However, I had a couple of occasions working in the school when after working beautifully for 20 mins or so in walk and trot, collecting, extending, leg yielding all quite freely and willingly, she was resistant to the canter aid, and "propped" after a few strides and reverted to walk, leaving me a little unbalanced. I thought she was going to buck - but she didn't. I did wonder if perhaps her back was stiff due to her age. Was she just telling me she had had enough, or was she picking up on my own anxiety? These thoroughbred types are just that bit more sensitive, and clever too.
The next time in the school, her owner watched to look for any signs of discomfort...and of course, she cantered perfectly and fluidly!
On hacks, she sometimes tenses up and prances before standing and staring into the distance - is this just napping, or is she uncomfortable in the cold winter weather? She doesn't do it when she is out in company, seeming to be energetic and free moving.
I am more used to geldings than mares, and I do also wonder if coming into season might affect her moods.
You might wonder why I have taken on this type of horse; her owner has two young children, and several other horses and ponies to school and exercise, so although she has had Millie since a 3 year old, she needs her to earn her keep, and is generous with hay and use of the facilities; also I need to know that someone will bring her in when the weather turns stormy, or feed her when I am unable to get there - so far it has been a mutually beneficial arrangement, especially when she can ride out with me.