Kaf starts at 14 and already amazing even back then, so does tuki who turns 17 recently. Of course we can't forget the powerhouse of today Ado as well who scream her way to stardom when she is very young. As long as they are not overworked its fine, I mean in the series we have Michiru as an example.
It's not just idol industry, in many sports, arts and even science many people are passionate enough to forgoes their health if it means for them to be able to breakthrough whether its for the position on the team, finishing masterpiece or seeing through the research they spend their life working on. Although in Tsumu case she is insecure as she is only now starting to found some semblance of success after being under her brighter sister for a long time.
