It is harder than is has to be if Scouting methods are not being used.
The mixed-age patrol only works when the Patrol Leader has been trained properly that his job is to be sure all the members are having fun in the troop and in the patrol's separate activities. As noted, "Servant Leadership" is a way to label the Patrol Leader looking out for the needs of of his patrol members first. When I was a Scout, it was "Take care of your patrol."
Now if this troop used age-based patrols, there would be a patrol of Scouts whose Patrol Leader would represent the youngins' at the PLC when program is being planned for the troop and, more importantly, would lead them in planning their own patrol activities that suit their interests, like a hike in the woods. In a mixed-age patrol, the Patrol Leader has the more complex job of insuring that everyone's needs are being considered where age spread creates more diverse interests and abilities. But even in an age-based patrol, interests vary.
Phrogger, you haven't mentioned any separate patrol activities, yet the Scouts are supposed to spend most of their time functioning as a patrol.
"[Patrols are] small groups of Scouts who camp together, cook together, play together, and learn together." "[The patrol members] interact in a small group outside the larger troop context, working together as a team and sharing the responsibility of making their patrol a success."
"Scouting happens in the context of a patrol."
"[The patrol is] the place where boys learn skills together [and] take on leadership responsibilities...."
"Patrols will sometimes join with other patrols to learn skills and complete advancement requirements."
And every single patrol member is supposed to be assigned a job that best suits his abilities and interests. That's part of the Patrol Leader's responsibility So it's a team - everyone playing a "position." He could perhaps be the geocaching coach for the patrol if he is really into that. Or the Game Leader who researches new games that interest the patrol members - more challenging with a mixed-age patrol but not by any means impossible.
Who is responsible for training the Patrol Leaders? Primarily that's the Scoutmaster's job, and it is his most important job after insuring that Scouting is a safe place.