Hm. Giggs was managed and Owen exposed in their early years. He needs rest time and recovery. Definitely.
Owen's just one example, though.
And Giggs wasn't particularly protected: he played 51 games in the 1991/1992 season, and 46 the next season. He was already a regular starter for us by the time he was in his late teens. Ronaldo was given plenty of opportunities at a young age, and flourished, without deteriorating too quickly. Same with Messi. Same with Aguero. Same with Rooney, even if he's looking a lot poorer now.
Some players wear out early, although that can often be more down to injury than anything else. Some players just naturally peak early. But the majority of top players generally start to get lots of opportunities in their late-teens. Martial should be no different, especially since he's playing well. I'd understand holding him back more if he was only 16 or 17, but at 19 he should be given plenty of playing time considering the hype around him.