13 is our cut-off. I never went for the "let's invite the whole class" parties anyway, so we usually had about 6-8 friends, cake, games, crafts, etc. But at 12 they get their last "theme" party. 13 issues in a new era, so they get to plan a more grown up activity.
This year DD13 decided she wanted to take a friend to Six Flags. We managed to find ultra-cheep tickets, so we could afford 3 friends

I had each girl make a sandwich at my house and we packed a cooler of drinks,water, and fruit before we left. We ate a couple snacks in the park, but went out to the car for our supper and rest break. DD decided she didn't want souveniers, she really wanted matching air-brushed tank tops for each of them--well, since we had already saved so much I was able to say yes!
The whole event cost less than $100 and has so far, been the Hit of the Season--the girls are
still talking about it!
Most recently I allowed a sleep-over with a twist--they had to set up the tent in the backyard and sleep on blow-up beds! DH rigged up the TV and a lamp for them, and the girls made monkey bread to snack on.

They were up partying until 1am, but they never bothered us. Next morning, they deflated their beds, cleaned up the tent and went home. Great party
