You've seen the leaks, I've seen the leaks, we've all seen the leaks, so here's my grain of sand.
Yes, the Zutara baiting is real, as is the not-so-subtle put-down on the ship. It's June and the Ember Island Players all over again...except that, when you consider the context, it's even better (in-universe), and worse (from a meta/IRL perspective).
But that's old news, and literally everyone is talking about Bryke and bait and "The Spirits Ship It!". And while I definitely agree with most takes, I'm here to say that the whole "An old couple!/NOT A COUPLE" interaction isn't what drew my attention, but rather the fact that they're both portrayed as elderly in the first place.
Allow me to explain: I don't know the plot of the movie and I don't know if the age changes happen because a) they're in the Spirit World, or b) something made it happen.
B) means that said something had a reason for making the changes like they did, even if it's just mischief.
A) implies that the Spirit World shows the true appearance of your soul/spirit instead of your physical age, or something of the like. Sokka's an interesting example: in the Spirit World, he's shown to be a teenager, not yet the man he's become in the physical world. He has so much learn still, so many things to do...and he's still stuck on his war years.
Now, Toph is a funny one. Which is honestly the best thing I can say about it. (I'm not yet fond of Baby Toph, in case it's not obvious. Or, well, I'm not a fan that they made it happen. Seeing twelve-year-old Toph again would've been amazing and far more in-character, IMHO.) And Aang...he's a young adult, closer to his current age than anyone else in the Gaang. Balanced.
So, if in the Spirit World physical appearance = spiritual age... Where does that leave Zuko and Katara?
Zuko, who has experienced more pain and life-changing events than most people do in their entire lives. Zuko, who had no childhood. Zuko, who was tossed into a world at war after having his innocence destroyed by his own father. Zuko, who saw the most horrid things life has to offer, and instead of succumbing to his worst instincts, he decided to bring life and honor to his people.
Zuko, who by age sixteen had the maturity and serious-mindedness of a much older man. Zuko, who has carried the weight of a nation on his shoulders since he was seventeen. Zuko, who tells horrible jokes and takes everything too seriously and whose love-language are acts of service.
Is it a wonder he's an old man, too tired to even stand on his own?
And then there's Katara. Katara, whose mother died to protect her when she was a young child. Katara, who adopted Kya's role in her family because nobody else could, because the void she left behind had to be filled. Katara, whose face overcame their mother's in Sokka's mind. Katara, who raised her brother, kept her family glued together, did chores, looked after the children, brought babies to life before she was even fourteen.
Katara, who found a legend and became a war hero. Katara, who continued to care for everyone else, mothering her friends and family. Katara, who never turns her back on people who need her, except for herself. Katara, who longed for someone to help her, support her, someone to share her burdens with. Katara, who tells horrible jokes and takes everyone's emotional burdens as her own and whose love-language are gentle words and acts of service.
Is it a wonder she's an old woman whose first instinct is to take over a baby's care?
I'm not really sure of what the thought process was when the writers made these decisions, but the fact that—when looking at each character individually—both Zuko and Katara can be defined as having a fundamentally old spirit... It says a lot to me, and makes them more compatible in my eyes than any "you look like a couple" moment ever could.
