Why would it take so long? Well, think about the kids in this experiment. They had time set aside for learning Morse code. Their "job" was to learn Morse code. They had someone there to teach and ask questions. They had others in the class to help them too, and provide motivation. An adult trying to do this on their own has no one to ask questions. There might be forums like this but they can be as toxic as they can be motivating. With no one else in a class to compare themselves to there's not much gauge on if they are doing well or not. Since adults will rarely get paid to learn Morse code there's plenty of distraction in their work and other obligations. I happened across something on a music school for people of all ages that teaches people in groups. The founder of the school found out that people learned to play more quickly if there were others learning with them. Now imagine this school compared to someone trying to learn to play an instrument on their own, using only YouTube videos and some books they bought. And do so while having to go to work every weekday as well as care for their children or whatever. You can't imagine? Well, maybe you need to work on your imagination.