I voted always. You stop paying for your kids cruises when you die lol. My parents still pay for my family when they invite us to go on a cruise, and I'm 58. They did, though, finally take me off their cell phone plan when I turned 54. Looks like with that example I'll be paying for DS 23 for a long time. And we've always been fine financially (good jobs, etc.) It's just something they wanted to do, thinking too they'd rather do that than leave us more money. // We've insisted on their not paying for us on vacations when we invite them (the trip is our idea and we are just seeing if they want to join us).
I really think a lot of it depends on your financial situation and whether or not you want to pay. We certainly pay for DS 23, are thrilled that he still likes to travel with us, and really enjoy having him along. Also, he doesn't make a lot, and we are comfortable including costs for him to go in the family vacation budget. It's a no brainer that we'll be paying for the cruise for him.
And, of course, don't do anything that doesn't work with your finances or that you don't want to pay for.