I teach several adults and actually find I spend more time planning and preparing for their lessons as they are usually able to work independently between lessons; in some cases they don't have lessons every week so they need even more work prepared. I do expect adults to set their own targets, but I also ask children to do this as well - they are more likely to achieve a target if they have set it for themselves. I see myself as more of a guide so I can show them the steps they need to take to achieve what they want. Targets range from things like "I want to play Jingle Bells in December" to "I want to do grade 5 at Easter".
As to learning from Youtube, I have several pupils who have self-taught in the past. Without exception we have had to go back to the beginning of a tutor book because they were missing essential basic information, such as the need to sit up straight or where a C is! I would say however that they have on average progressed much quicker than those who didn't play before, but they could have saved the frustration of having to start again if they had had some lessons in the first place.