You're probably overthinking this a bit... maybe more than a bit... but I'm often guilty of that myself, so I get it.
If you look at the really high end or specialty cruise lines (think Regent, Crystal, Oceania, Cunard), then maybe. I think those lines do cater overall to an older crowd, and possibly you'll see the impact of that on those lines. But the cruise lines you've mentioned are in a completely different category. All are what I would consider to be "mainstream", and the demographic differences between them are going to be minimal (except on Disney, where you will definitely see more kids and families). Those lines might see an older skew on certain itineraries, but they're not going to significantly change their activities just for those particular cruises. If you've sailed on them before and were happy with the activities offered, it's not going to make a difference if the average age is 5-10 years older on the Baltic cruise. Much the same activities will be available.
If you want to avoid kids, don't sail during school vacation times. It's really that simple. But if you do that, you should certainly expect an older crowd overall, no matter which cruise line or itinerary you choose. That's just the way it works. 20-somethings really aren't a big cruise demographic, because most of them don't have the time or the resources to cruise frequently. Of course there are some, but if you take out the families with kids and the seniors, there just aren't that many people left over to fill a ship. So you'll kind of have to pick your poison.
If you really want a younger, non-family crowd, the closest you're likely to get is probably Carnival, at least by reputation (I haven't sailed with them, so can't say firsthand). But that wasn't one of the options you're considering.
In the end, I think you'll be fine on any of the lines you mentioned. Best of luck to you, whatever you choose!