I really liked the Shazam/Jon Kent issue. It sort of hit my emotions specifically.
I have this thought experiement I run sometimes, where I imagine being a teenager again, but with all my adult memories. This thought experiment mostly just depresses me. I've concluded that I would be forced to relive painful challenges I've already overcome while losing all of my years of hard work and potentially all of my dearest relationships. That one hits me really hard, especially where it concerns my husband, my person, my Jay. It's hit a point where I now imagine him also be transported back so we can find each other. I can't easily imagine living without him, and know with all my heart that we'd both do better surving anything together.
Maybe you can see where this is going.
Damian has it right that Jon being de-aged is depressing. That's because Jon being a child again forces him to lose his life again. Yes, he wouldn't have to be so responsible anymore, but he would still have the mind and heart of someone who does. And suddenly, he wouldn't have the powers or authority to back it up.
Jon aged in real time, but not in sync with his home universes time. It's not like Jon was suddenly aged up. He didn't go to sleep at 12 and wake up at 17. He was isolated and tortured for years before he escaped and made his own way before finding his way home. He grew up, just not with the people he loves most. At minimum, that would have to put a strain on relationships, as people expect him to be one way, a kid who's still figuring things out, but he's different now and an adult who's self-sufficient and has been for a while.
To go back to that younger age would mean straining those relationships again in new ways. Suddenly, his powers he could depend on wouldn't work right anymore (something which could easily be triggering, given those years in the Volcano were because his powers couldn't work), and he'd be legally a child. It's not easy to feel like someone is treating you like a kid when you aren't. Imagine how badly it would hurt to be an adult stuck in a kid's body. (There's a whole BTAS episode on that.)
And most importantly, Jon would have to give up his relationship with Jay if he remained a child. Jon even states that with everything that's happened, all those years he's fought through, that the one anything that's made it okay was Jay.
I didn't call my husband my Jay as a joke. When I can't trust in anything else, even myself, I can trust him. And vice versa. Jon calls Jay because he believes he can help and actually make it better, even if Jay can't find an immediate solution.
When Jon goes to Damian, it's less for help and more Jon wanting to try to play the way he did before, but Damian's older too and wisely chooses not to try to relive the past. When Jon goes to Jay, it's for his partner, his support, his person. And yes, Jay is able to look at the situation from the outside and see what Jon and the others can't. (Which is something I think Jay and Jon have both provided for each other in the past multiple times.) He fixes it, but I don't believe Jon called Jay simply because he thought Jay could fix it, instead because he needed his pillar, his focus in the storm, his person.
And I love at the end, the pair of them making a pillowfort and being silly, but also choosing to go forward with their lives.
Nostalgia has its place, but it can blind you to the future. There's a place for enjoying those things you loved as a child because those things are still part of you. But as an adult, there's a lot more years you'vd lived, and so there's a lot more that makes up who you are.
I was reminded of the Sailor Moon Super S movie, where the scouts have been turned into children at some point. They're talking about how nice being a child is, but Neptune says something to the effect of "but there are things adults can enjoy that children can't", which given the way Uranus blushes is definitely a sex joke. And yeah, obviously there's that, but there's something quite wonderful about being an adult. There's a beautiful complexity to being an adult and getting to decide what that means. And there's a different beauty to being an adult who is with another adult and both of you getting to decide together what types of adult people you're going to be.
So yes, I loved the last part with Jon and Jay, the recognition of both of them needing those light-hearted, childish moments, but also both desiring to continue to be adult men with each other. Them finding that balance together is really beautiful, I think.