And what about getting a job? When you're finally done with college, you'd be a year behind your peers. Many people would rather hire the younger person if given the choice between, say, a 22 or 23 year old.
I think you may be thinking too much in terms of high school in which each grade is of the same age group and anyone who is older is seen as somewhat different or behind. College/university are completely different environments. I go to a relatively good university and I would say that the student body consists of students aged between 17-40 years old. Roughly 40% of students have not come directly from high school. As others have just said, you interact on a daily basis with a diverse group of individuals - which is exactly what you'll be doing when you enter the workforce/'the real world' too!
You'll also find that these days a large majority of graduates will return to study, either to further their current degree or to study something entirely different. So starting a career can often happen much later than 22 or 23.
Speaking from only what I know, with most Australian universities, every student who is accepted is given the option to defer their placement for one year. On top of that, there are also a significant number of places specifically reserved for those who haven't come straight from high school and who didn't defer their first offer.
As far as deterioration of study skills, I went straight to university from high school but I did end up taking a year off last year due to health reasons (between my 2nd and 3rd year) and on my return I honestly had no problems getting back into the swing of things. But, you know, everyone is different, and if you feel you may need a refresher I am sure that most universities offer free workshops and support. As far as my own goes, it offers free workshops during each semester on time management, stress management, essay writing, oral presentations, exam prep, note-taking, etc, and there are numerous resources on the university website if you aren't available to attend the workshops physically. (It really can't hurt to have a refresher even if you did come straight from high school anyway!)
But that said, I personally feel that not only is university a different social environment; it is a different learning and teaching environment too. So I think in one sense you will develop a new, perhaps more individualized, set of skills anyway.
Admittedly, on my year off I did become rather lazy. I think because time just goes
so fast when you don't have a strict regime that schooling requires. So I do agree that planning is VERY important if you want to make the most of your time off