Let's (cohort) party down

A friend of mine recently sent me email in which he said he'd been standing in an elevator with some young people who were talking about a "cohort party" and did I know what that was? Not me. To my surprise, searching around revealed many uses of cohort in casual ways that suggested people were familiar with the term:

• If anyone is interested in having a cohort party, feel free to suggest some possible ideas. […] On friday, Melanie was saying something about a get together on December 15th, which is the day after we finish our practicum [#]
  
  
• Some girls from my Elementary Education cohort decided to have a couples party. [#]
  
  
• Each intake is split into cohorts. Each cohort divides into clusters. And each cluster… well, it’s just a cluster. So a lot of socialising happens at the cohort level. Like last night at the Irish pub in Rittenhouse Square, where the INSEAD group crashed a two-cohort party. We were told to ask the cohort of whomever we spoke to before they had a chance to ask ours. If they said “cohort E”, we were to pretend to be from “cohort I”. If they said “cohort I”, we were naturally from “cohort E” [#]
  
  
• We have had the chance to meet many members of Cohort 10 as they’ve joined us for classes and speaker series over the past few months […] Now we are all anxiously awaiting the Baltimore Study Group-sponsored cohort party in January. [#]

And so on. But no actual definitions, as in "a cohort party is ...," really sprang out. My kids are college age and not un-hip; when I queried them, my son did note that he understood the term cohort in its, what, sociological sense: "a group of people having approximately the same age." But the term cohort party rang no bells with him.

I have to conclude two things. Thing one: I'm an old guy and the term cohort seems sort of quasi-technical to me (when I hear it, I think "academic paper"), but for younger people, it's a normal word that they're used to hearing in descriptions of their class/group/work unit. I asked Ben Zimmer about this whole thing, and he noted that he only started hearing cohort when he got to grad school, where it was used to describe what I might have termed his "graduating class." An n-gram for cohort provides a bit of evidence that use of the term has been rising since about 1970 or so:

Thing two is that a cohort party is nothing more than a party that your cohort -- your school class, whatever -- is throwing. And the reason that this seems strange to us oldsters is that cohort is just not a term we were brought up with.

If anyone knows something more specific than this, I'd sure love to hear it.