How does age matter in education?

As I write my house is full of kids--three of my daughters (ages 12 to 18) and four of my nieces and nephews (ages 2 to 8).  I am the only adult here.  I've been watching the kids interact, and seeing the mutual pleasure they find in being together.  Much of their play is about learning--how to play a particular video game, how to make lunch, how to keep track of each other, how to make sure that everyone is having a good time.

Their presence (and a recent conversation with my wife where she pointed out that we are potentially grandparent age (since two of our own children are now over 18) and that she would welcome the presence of some little kids around the place--yikes!) has me thinking about age and what it means for learning.

When my kids were smaller they went for a year to a non-graded charter elementary school called Sundance Mountain School.  (It continues to this date, though under a new name, Soldier Hollow Charter School.) I was never sure how good the learning was in a formal sense, but in terms of practical experience, there was something wonderful about 5 year olds and 12 year olds learning science and math together.

In this now-famous talk, Sir Ken Robinson makes the point that among the industrial-era absurdities of schooling is that students are grouped according to their "date of manufacture" rather than some more educational commonality (or difference).  It is among the practices that squashes the creativity out of people.

It seems like colleges would be a place where we could learn about the roles of age in learning.  After all, most classrooms include students with different dates of manufacture, and particularly in schools where there are many non-traditional students, the age gaps can be quite significant.  But there is no reform movement or pedagogical approach (that I know of) that attends to age (with the possible exception of freshman learning communities which group students by age, sometimes to the frustration of faculty who think it makes the classroom "too much like high school.")

Is it the case that once a person reaches, say, 18, that age no longer matters and therefore we make nothing of in in higher education?  Are we losing potential learning by just assuming that the age system (which, for example, mandates that you have to be between 17 and 19 to start college) makes sense?  Is there some reason to accept things as they are?