I actually wrote a blog post
about this today!
I'm a bit surprised to see people here saying that disney pushes its cruises to adults, I've not seen anything which indicates that.
As for adults who go on them: What's the harm really? If they like Disney and they're willing to spend their cash on it then what's the problem? I'm 23 and would kill to be able to go on a Disney cruise ( I'm basically a 10 year old at heart).