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.
I know how hard it is to lose animals like that, and I freely confess that I dread the day that time arrives for my Huey. And that was something that was a big factor in my decision to buy him last year, when he was 17...
...and it stayed a big factor until I realized that young horses, adult horses, middle-aged horses, elderly horses - all of them - can get terrible injuries, or catch horrible diseases, or colic, and have to be put down even if they are young.
I think that any time you decide to own a horse, you have to confront that fear, or worry, or nervousness about possibly having to put your steed to sleep and losing a best friend. And it's not just horses - it's about 1,000,000 times worse when you're having a baby, or when you get married. The ultimate loss is always part of the deal from the very beginning. It's life. If you want that rewarding relationship, you take on the risk that you'll lose it all. In fact, it's almost guaranteed.
But we do it anyway, knowing - at the beginning - what the end will be. There's no surprise plot twist, here. No person, no animal, gets out of this alive, and unless that person or animal has led a sterile, cold, and solitary life, someone is going to miss it tremendously when it's gone. It's painful. Even when you know what's going to happen, and you prepare yourself for it as well as you can mentally, it still hurts like heck, and that loss still leaves a hole in your life. It always will be there. Hole hurts less after a while, but it's still there.
So, I am VERY sorry that you lost your other horse, and I know very much how frightening that is, and I applaud you for being willing to enter into that kind of relationship again. It means you are a strong person.
What I'm saying up there is scary, yes, but it can also be a source of strength. It is entirely possible that your yearling will die, or have to be put to sleep, before your middle-aged work-horse. It is not a question of "will you experience that pain" it is a question of "when will you experience that pain". You're already in that water with both of these horses. My advice is to just dive right on in. Let your older boy have your heart, right along with the yearling. Love him, ride him, challenge him, keep his mind active. You may still have another 20 years on him. There are two horses in my barn that are nearly 40, and they're both still trucking right along.