I think mixing is fine. Some schools skew towards older and prefer applicants who will “fit” better on that front. Some may have enough offerings to consume 4 years. (This so depends on interests and could be anything from study abroad to a high level of a foreign language to advanced math.)
I 100% agree that it should not be about "easier admissions " and that any applicant considering this should be asking if it is worth it for them. It’s hard to put $ to some of the benefits but thinking through it, you may develop a sense of it.