Many professionals do take decades to develop their process and it's not uncommon to open your first fund (or to get your first independent PM gig) in your 50s. In the end, professional trading/investing is about developing a process and refining it. Some people get there quicker and some people get there slower, but developing a good process takes time.
Here is a typical institutional progression. Usually, you start as a junior trader (buyside or sellside) and learn the ropes for 3-4 years. During that period you get relatively little risk-taking responsibilities of your own and mostly support the business. Then you move up and start taking risk of your own, slowly developing your own approaches to the world. Give it another 3-5 years. So there, you have about 6-10 years of professional development only to get to an early stages of independent risk taking career. Now combine this with the fact that most investors or fund managers want you to have experience in multiple market environments and we are taking decades before you "have made it".