It used to be you did your time in the bullpen before you became a starter, if you throw too much before you reach the age of 25, you’ll pretty much blow out your arm, but after the age of 25, if you haven’t thrown too much, your arm will pretty much be immune to those kind of injuries, it’s why Nolan Ryan had the career he did, he started in the Mets bullpen.
Studies have said that you can increase a pitcher’s workload 20-30 innings every year. But teams won’t even do that.