I guess it depends on what you want to accomplish, how easy it is for your to pick up new ideas, how adept you are at being self taught, and how quickly and in how much depth you want to learn to sail. There is no doubt that given enough time most reasonably intelligent people can learn to sail to one degree or another without the benefit of a formal course. If you have the apptitude for self- education for sailing, your progress can be reasonably quick. BUT in general trying to teach yourself to sail is the slow way to go and there are many things that you may never be able to learn on your own.
Courses are a good way to develop the basic understanding of sailing. I have been quite impressed with Steve Colgates School at least from conversations that I have had with recent graduates and from the printed materials that are from the Colgates. In many areas of the country the US Power Squadron and US Coast Guard Auxillary offers high quality navigation and boat safety course.
By the same token, courses can only teach so much so to really learn to sail well you also need a lot of reading and a lot of time on the water. If you are in reasonably good shape and you care to accellerate the learning process, nothing teaches the finer points of sailing than sailing a dinghy
or responsive small keel boat (less than 28 or so feet with less than 26 feet being more ideal). Another way to accelerate skills building is to do a bit of racing with a knowledgeable crew.
You just about can''t learn to sail well on a cruising boat that is much over 30 feet or a boat with wheel steering. There just is not enough feel to develop skilled boat handling and sail trim.
That said, not everyone needs or wants to learn to sail beyond the basics. There are a lot of folks out there who get by on comparatively poor sailing skills. There is no one right answer here. I will say that in my experience, as you get older better skills are required because you can no longer count on being able to manhandle the boat like you could in your youth.