Around 18...
I'd like my friends to actually dance, usually at parties they just sit on a couch and do nothing.
A lot of "parties" end up like that, so that you could say they get altered into more of a "get-together".
It's funny how when certain people would "throw a party" there would normally be dancing and shouting etc. and at others there just wouldn't be, no matter how much music was played or what kind.
It wasn't even down to the people themselves, as some would invariably sit at one place, but be willing to dance at others.
You need a fairly big place with lots of center room if you're going to have much dancing going on, too; which is always another consideration.
( Sometimes the furniture is moved elsewhere, but that sounds suspiciously like "work" to me.

)
A lot of people are shy, too, and so they wait for others to "get things started" because they don't want to be the first one on the dance floor.
Oftentimes it is the people who generally don't have a lot of ( thoughtful or otherwise ) things to say who kick the action off, as they may be very good looking, "connected", or otherwise influential types. Or it will be the exact opposite kind of people ( getting back to my remarks about the artsy crowd in an earlier post ) who affect a punk sensibility, and purposely dress "nerdy' etc. who will function as the initial "tinder" for the fire, so to speak.
It's always tough to figure out the timing. The party has to have some kind of "start time", especially if it's one that was planned. ( There are some places where everyone knows the parties are taken for granted to happen every weekend, regardless, and no "invitations" etc. are needed. ) Yet if it has a start time, there will be the people who like to arrive "late" as its considered "uncool" by some to be amongst the first ones there. If some of the tinder folk arrive before there is much of a crowd, they will assume that this party is "dead" ( or whatever kids are calling those now ) and move on elsewhere before it ever even got a chance. Yet if you say to people "Figure on showing up around 1" people will look at you like "PM or AM, dude ? "