A lady once told me that a younger horse may pick on a much older one because ......say in a wild situation a much older horse would tecnically slow the herd down there for a younger one would chase it away to get rid of it.
I witnessed similar behaviour when I ahd my first horse, he was like 21 and had arthritus. A younger TB of about 6-7yrs was a real meany to him, chasing him and biteing him
So we ended up seperating them as it was too much for my boy with his arthritus.
Your horses don't have as big of an age gap but it could still be similar reasons.
Either way I think it might be better to seperate them.