It must depend on how they’re trained and what is the parish’s expectation and tradition.
We have kids here every bit as wild and woolly as those described in this thread. But there’s NO WAY they would be tolerated behaving irreverently as altar servers. We have boys and girls, serving together, and they are neither soldier-like, nor, at the other extreme, fidgety, slouching, whispering or giggling. They just carry out the job quietly, reverently and gracefully, because that’s the way they’ve always seen it done, and they know that’s what’s expected of them, too, as altar servers. No one is forcing them to be servers, but we always have plenty, and in many cases they continue to serve until about the age of 14 or 15. Yes, they do wear sneakers under their robes, and heaven knows what else in the way of clothes, but, personally I can live with that because I am happy to see these kids involved with the Mass, and they all do a great job.
Outside of Mass, they look like any other kids.
I think it’s up to those training them to let them know what behaviour is expected, and once a “visible tradition” of this is shown long enough, the newcomers will follow suit, because they SEE that that’s the way it’s done in the parish.
And by the way, reverent altar servers abound not only in my parish, but in the whole diocese, and anywhere else I have attended Mass in New Zealand. (As I’ve said elsewhere, secularism is more rife here than in the U.S., so it’s not that kids here are more religious. Maybe more disciplined in the home, or school, or something

?)
A Te numquam separari permittas - never let me be separated from You