Maintaining a show is a difficult thing, and, I agree is becoming a lost art in theatre.
1) In most small regional theaters the director (usally the owner of the company) is almost always on hand to watch over his/her *baby* and is giving notes on the sly.
2) Econmically, alot of theater's can not afford to run a show longer then 3-4 weeks, unless it is a smash hit money maker beyond the subscriber base audiences.
Show maintenence is not something that is really taught in grad school: the runs of shows are way too short to really need to concern maintenence. We had some discussions about it in stage managment class, but what I have learned has been on the job from trial and error...lots of error.
In my experience, giving notes and show maintenence is not about age, it is about trust. If the actors can trust that you always have correct answer and have a complete understanding of what is going on, they will listen and in most cases (the rare diva excluded) take the notes you are giving. You, as the stage manager, have to make yourself apart of their ensemble. When you hold a show and are calling it, you are an invisible character on the stage with your cast. I have often been the youngest member of an ensemble, but been given ultimate trust because my actors trusted me to make the right calls when scenes needed a tweek because I have been attentive to the entire process. (FYI-it take practice and experience. I really didn't feel confident in my ability to maintain a show until after 5 or 6 years of stage managing and I was lucky enough to work with a director who would point out to me good and bad growth in a show and give me the insight to fix it without violating my authority as stage manager.)
Here is what I do:
1) I write down intentions, emotions, and line readings in my script as well as blocking so I can be aware of the emotional paths as well as the physcal paths of the actor. That way I can give emotional clues to the actors as well as physical cues when they get *lost* in rehearsal. Actors tend to relate their blocking to the emotional. This also helps establish that I am paying close attention to them, building trust.
2) I always attend the actor's notes sessions and listen to the notes the director gives the actors. This always gives me insight on the types of things the director is looking for and how he/she is tailoring the show
3) I always have a conversation with the director prior to the final dress rehearsal and discuss a formal "handing over" of the show during the finaly notes session. I ask the director to say "the stage manager is now in charge and any performance notes will now come through her." This establishes a clear line of authority for the actors. I also let the director know that I would love to get any notes they have about show maintenence throughout the run so I can pass them on. At this final dress rehearsal, after the directors announcement, I clarify with the actors that I will be giving the notes and taking questions about the show that they may have. I also set guidelines for notes, for example all notes are given prior to 15 minute call or will be posted by sign in sheet.
4) I also discuss with the director the types of "growth" they want to see as the show settles so I can watch for it and reign it in if the growth happens in the wrong direction. For example, an actor morphs a funny line into a slapstick moment that pulls focus from the story of scene. You can always guage an actors growth with the deeper/wider theory. If the role becomes more meaningful and honest, telling the story, it is growing deeper (good growth). If the role becomes bigger, exaggerated and superficial (ie...playing for the laugh) that is wider (bad growth).
Best of luck!