It's the normal order of things. The older horse will eventually give way to a younger herd boss.
My 27 y/o has been the herd boss since 2007 when my beloved heart horse had to be put down. He was second in command to Conny until then. Now that he's getting up there and slowing down tremendously, my young TB is starting to vie for the head spot.
Completely normal, although it can be kind of sad to watch.
Last year my vet asked me if my Alpha was starting to turn over some of the herd duties to Horse #3. Astounded at how he picked up on that, when they were all in their stalls, I said that he was.
The Alpha Guy is 25-1/2, has metabolic issues and hock/ankle arthritis. While he is still the boss and everyone respects/listens to him, he is allowing #3 horse to have more and more leadership. Unless there's an impending storm then everyone had better listen to #1.
The #2 horse is what is called a Passive Leader; he just turned 27 and never has had the desire to lead the herd. His mission has always been to be a good listener and offer comfort when needed. He is my best nurses aid when I have a horse in the sick bay yard.
It is, indeed, sad to watch the shift of power. Especially when the Alpha Guy has been my heart horse for 22+ years. He is the handsome fella in my avatar, when he was 16
The Fella in my sig is my 27 yr old Arab who is the Passive Leader; he was 23 when that picture was taken:)