These day we more often see actors in their younger ages.
Do they get paid for that or can companies just model whatever they want with no need to pay to the model's prototype?
the tl;dr is that it's complicated. A lot of it depends on preexisting contracts and legislation, but a lot of laws are lagging behind the advancements being made in technology. Some studios and directors acknowledge actor's rights to their likeness and are respectful, and others don't care.
Screen Actor's Guild on digital image rights