When your play is published, it gets listed in the publisher's catalog, which may be read by thousands of theater groups. Every time one of those groups produces your play, you receive royalties and know that your play was performed so many times. You may even find out who produced it and where. But you won't get to see most of the performances or be involved in the casting.
About once a year, I get the opportunity to see a play of mine performed by some group where I am not involved with the production. I'm generally pleased with how the show is cast, but sometimes don't agree. Maybe that director had a different interpretation of the character, or maybe the director understood my original intent, but there were no actors available with the specific look I had in mind.
Early in my scriptwriting career, I learned that flexibility in casting pays off in the long run by allowing directors more leeway. Many times, I've seen shows passed over because the strict casting requirements were more than a group could handle. Most of my plays are published by companies whose clientele range from middle and high schools to community groups comprised of relatively young (20s and 30s) actors, to professional groups who can hire actors of any age.
Children, particularly, may have difficulty in portraying some adult parts, both for appearances and for the lack of experience from which to draw upon for their inspiration. We've all seen examples of a 12-year-old boy with a fake beard and moustache and a much-too-large suit, attempting to portray an older man. Or the teenaged actress with makeup of diffused grey tones and thick black age lines attempting to look elderly by hiding her naturally youthful face.
This is not to say such scripts shouldn't be written, but rather the scriptwriter should consider that some time in the future, roles may be performed by people you might consider inappropriate, but which that director believes to be perfectly suitable.
This is the acid test of a script. Can the director and cast, from the script alone, produce a show so that your original intention of the script comes out, the cast and crew enjoy staging the production, and the audience enjoys watching? When I see a show of mine done, and can answer yes to those questions, that's a successful ending.