It’s okay to euthanize when you can’t keep a neurological horse comfortable.
I euthanized mine last fall because I knew he would be unhappy and uncomfortable in the cold. I also felt the risk of him falling was high enough that I wasn’t willing to risk it.
There’s no timeline, or age, or minimum amount of money spent before euthanasia is permitted. Just because the horse is doing all the horse things like eating, rolling, etc and isn’t limping, doesn’t mean the horse is comfortable or pain free. You don’t have to wait until they’re limping or incapable of getting up. If you, the person who loves him and knows him best, can see that he’s not comfortable and you can’t help… it’s time. You’re not giving up on him, you’re freeing him from his pain.
Don’t wait until the person who casually looks at him as they walk by his paddock can see it.