If you are continually schooling in an arena setting, he is likely bored to the hilt and is becoming dull. He needs time out of the arena on the trails, perhaps with others. When horses get a break from repetitive work they often come back to it better than before. There was a cutter who had competed at many shows and was holding first by a good margin but his mare was getting dull. He took a chance and took her home for a month to laze in pasture. When the finals came she cut with brilliance, better than she ever had and they won the NA championship.
I personally don't think 19 is old either as I have stated. I used to have a 25 year old mare.
I am thinking it is because he is not as worked so I'm hoping the more I can get out there hopefully with more riding he will become less lazy (which I believe is the issue)
He is completely healthy and UTD, he does not have any problems except for generally just slowing down in general.
I currently cannot take him out of the arena and he has only been ridden in it twice, due to the fact we have things that need to be worked on, and he will spook to easily outside, so I would like to take it slow with him.
I mostly just have Demitri in pasture with the group at the barn, and he gets plenty of time off.
It also of course doesnt help that currently we are going through a heatwave, But I am hoping as I increase his workload and bring him more up to fitness, things will be going better, as I wouldnt like to have to retire him until a year or two or so. I will probably be going out on monday to see him, in general though 'for his age' as others have said, he is doing good.
I'm also really not used to more lazy horses either, so it could be my problem and i'm just THINKING he is slowing down and everything, I've always ridden thoroughbreds, or a hotter breed.
So I am thinking, its just a fact of bringing him back into work and challenge him consistantly.
EDIT: I also want to admit, that I am nervous of owning older horses, due to the fact I have had my 21 year old pass away last year and that literally broke my heart, I have to say, I am afraid of the same outcome.