The guy who plays Hannah's older brother (around 15 or 16 on the show) is around 30 in real life!
If you think there's no discipline behind the scenes, think again...it's just very closed-door and not at all publicized like the infraction would be. And of course given that these are (mostly) child stars, they get punished (1) by the production (2) by the family and (3) by the media sting and having their dirty laundry in public...granted maybe in each case you may not have all three (in some cases for example, the family may not even care), but fame comes with its prices and you can take it to the bank that the kids pay! Big! One way or another.
And yes, stars can be replaced. Speaking of the guy who plays Hannah's brother, he had a close call...after doing a scene that required several takes, he engaged in some alchohol on set and a bottle was left on set which Disney later had to digitally remove after fans caught it! He came very close to being written off after that.