Again it's a cost-saving measure for productions all around as if you employ someone who is under the age of 18 the production has to:
- Provide an on set tutor, and set aside time for the tutor to instruct the minor. (Because they'd be in school if they weren't working.)
- The Parent or some other legal guardian has to be on set at all times when the minor is working. And they can raise objections or offer suggestions as to how the minor should be treated/handled on set during production.
- There are hard set in stone limits as to how long the miner can be allowed to work on set; And there is a break schedule that must be strictly adhered to (even if they're in the middle of shooting at a scene).
So yeah that's why any production will try to get away with someone who's 18 or older, having them play someone that's much younger than their actual physical age. These days the only time you really see a child actor on set is if they need someone for a really young character (like 12 or younger).
Once you're an adult, the production can pretty much treat you as they please; which is much more cost-effective for the production.