Sad but true. I was like 12 and I'm 24 now.
I want to play it, but not without having the previous games and it just feels like such a tall mountain to climb at this point to play the whole franchise
This one has all the games.
The most essential ones story wise are Kingdom Hearts I, Chain of Memories, Kingdom Hearts II, Birth By Sleep, and Dream Drop Distance.
But since the gameplay isn't too great in games like Chain of Memories for example, you can watch videos summarizing the story for some.
I'd say you can get by with just knowing the story for KH1, KH2 and Birth By Sleep, and then watch the secret endings for Coded and DDD on Youtube and you'll be pretty caught up.
If you want to focus on just playing the ones with the best gameplay (imo) then play KH2 and Birth By Sleep.
I have been waiting so long for KH3, so I am extremely excited for it! However, if it is bad then I probably won't trust Square enix much moving forward.
As a side note, something has always puzzeled me. Why don't these games come out on a Nintendo console? I mean rpgs typically do well as do games that appear to be for young audiences. KH fits both of those categories.
There are a bunch of them on Nintendo handhelds.
As for the home console entries, I'm guessing KH1 and KH2 weren't ported to Xbox or Gamecube because those systems didn't do so well, while PS2 was doing extremely well. After the success of 360, SE announced KH3 for the next Xbox system. As for WiiU and Switch, I imagine they'd probably be very difficult to port KH3 for with acceptable results. There's no way SE would prefer Microsoft over Nintendo for a game like this otherwise.
I suspect it will sell rather poorly on Xbox One, but I really hope I'm wrong.