Recently I was asked about "Adult Learning Theory". There is a free wikibook "Learning Theories/Adult Learning Theories". The "Andragogy" Wikipedia entry is useful. The word Andragogy, is clumsy: intended to distinguish theories of adult learning from learning by children: pedagogy. But I am not convinced there is a difference between child and adult learning. In particular the current practice at many universities where undergraduate students are treated more like children than adults in not a good approach.
The approach of "Adult Education" is to assume the student is studying part time and has a job or other activities which occupy much of their time. Also adult eduction assumes the learner will take more responsibility for their education, being more self-motivated and disciplined. This is also assumed of postgraduate university students.
However, my experience of being an adult postgraduate student was that I still needed structure and guidance. Some universities are claiming their students have graduate attributes, such as multi-disciplinary analytical skills, clear and logical communication and teamwork. But students need to be guided to obtain these skills.
The idea that undergraduate university students are full time, on-campus is not the reality. Even "full time" students have jobs or other commitments. Rather than create an artificial distinction between full and part time, on and off, undergraduate and post graduate, I suggest assuming all students will have a range of needs and capabilities. Those students who need support should be able to get it when they need it for the part of their program they need it for. Where a student can manage on their own they should be allowed to.
This would be much the same as the approach take to access for the disabled on university campuses. Rather than assume that students don't have a disability and then make special arrangements for those who do, the opposite is done. It is recognized that most people will have some form of disability some time in their life and so the campus and courses are design to accommodate a broad range of abilities.
Buildings are design for wheelchair access (which incidentally makes them easier to access for everyone else). Course notes are designed, by default, for those with limited vision (which also improves their general readability). New buildings and notes are designed to be accessible because this is cheaper than modifying them later (and is required by law).
In the same way I suggest that university courses should be designed to suit a broad range of students. To the extent possible, the students should be assumed to be distance education students who never visit the campus. It should be assumed the student can, where possible, do the course in complete isolation from other students (such as the extreme case of where they are in a prison cell or a fox-hole on a battlefield). But students should be given the option of human tutor help and group learning. What each student needs will vary from subject to subject and time to time.