Inspired by Latecomer's threads on the subject as it pertained to G4. In fact, to quote him "I know - who'd want to give up diapers? But it's clear enough from canon that most ponies (and other cretures) eventually do."

Obviously, our main characters (and several others) of G5 had to undergo it at some point. Heck, even Sparky presumably is potty trained since we've never seen him wear diapers. So go ahead, feel free to speculate about the subject. Maybe the pony tribes have different customs as it relates to potty training? What (if anything) might have changed between G4 and G5 in that matter?

Here are some of my thoughts, in case anyone's interested:
Sunnny Starscout: She was clearly out of diapers at the beginning of A New Generation, as were Hitch and Sprout. But from what little we know of Argyle as a parent, I wouldn't entirely be surprised if maybe he was a bit slower than most other parents when potty training Sunny. He probably wasn't bothered if she maybe stayed in diapers for longer than most ponies in her age group. Of course, he knew he'd have to get Sunny trained eventually so she could attend school and so that she wouldn't be teased and made fun of by her friends. Sunny's training, whenever it did occur, was a largely uneventful affair. Or, perhaps her training occurred before whatever happened to her mom happened.

Hitch Trailblazer: Given how much of a "model citizen" he tries to project himself as, I could easily see Hitch being among the first of his peers to get fully potty trained. It may even be that his training is what instilled in him at an early age to obey the rules and laws of society, written or unwritten. As to whether or not he regrets getting out of diapers so quickly, that depends on how old he was when whatever happened to his parents happened. "Winter Wishday" suggests he was in Grandma Figgy's care since at least the time he got his cutie mark. But the time before then is a mystery.

Izzy Moonbow: With the canon suggesting that she was an orphan raised by Alphabittle, and that aside from that she had free range of Bridlewood, Izzy's training was probably much more of a hassle than it had any right to be. Of course, now that we know Alphabittle was already a parent before he met Izzy, he likely had experience with potty training and may have been the biggest contributing factor to Izzy eventually getting out of diapers. With how run down Bridlewood seemed to be in "A New Generation", they perhaps didn't have indoor plumbing. They may have had outhouses, latrines, and perhaps even chamber pots. That probably further complicated things with Izzy, getting her to sit still and understand that she couldn't go to the bathroom wherever and whenever she felt like it.

Zipp Storm and Pipp Petals: Seeing as these two characters shared a parent (Queen Haven), their training was likely intertwined with one another. Whether their father was around at the time, or whether Queen Haven was a single parent from the moment of Pipp's birth is up for debate. Canon suggests that at least since Zipp and Pipp were fillies, Queen Haven has been a single parent. Zipp at least started her potty training before Pipp since Zipp was the older sibling, but given how independent Zipp was shown to be throughout "A New Generation" and how distant she seemed to be from her mother, maybe she was not as quick to fully master potty training compared to Pipp. Pipp's obsession with phones and social media might have started during her training, when Queen Haven would give Pipp a phone just to keep her occupied whenever she was on the potty. And more often than not, various castle staff includes maids and guards would have to be pressed into service to help deal with various aspects of the training, including cleaning up after any "accidents".

Misty Brightdawn: We know she was with Alphabittle until she was at least a filly. And whatever sort of plumbing that was around when Izzy was in potty training probably was around for Misty's training as well. Alphabittle, like Queen Haven, may or may not have been a single parent when Misty was little. And we know he was younger during those years. Misty's training is an open ended book, it could've gone as easily or as poorly as you think it did. There's nothing in Misty's personality to indicate one way or another what her earliest years might have been like. But of course, whatever experience Alphabittle had with Misty, it unknowingly helped him prepare for when he had to potty train Izzy.

Sprout Cloverleaf: He's been shown to be a real mama's boy, and even after his supposed "reformation" he's still very immature and still houses his speciest beliefs about the unicorns and the pegasi. Depending on what his family situation was at the time, his training could've been routine or it could've been delayed. If it was up to his mom to train him, I could very easily see Sprout not so secretly persuading her not to go through with it. And as a result, Sprout lagged behind most of his peers in being fully trained. He may boast that he was out of diapers before Sunny, but secretly she may have actually beaten him by just a few weeks or so, not that he'll ever admit it.

Sparky Sparkeroni: I refer you to a comment made on my fanfic "A Sparky Sprout:

but I suppose, for all I know, Sparky was already potty-trained straight out of his egg.

Either that, or potty-training him quickly became Hitch's priority number one shortly thereafter.

If Hitch did have to potty train Sparky, I imagine that was probably quite a hassle for Hitch since he couldn't understand Sparky, and Sparky might not have fully understood Hitch. The training probably involved a lot of trial and error.

Posey: With fussy she tends to be, and given how much she didn't like the introduction of magic back into Equestria, she's probably one of those individuals who was potty trained at an early age, and the process was done to the point where it adversely affected her. Depending on if toileting customs changed in Maretime Bay during her lifetime, she might have had difficulty adjusting to the new standards. But she'd probably be far more likely to complain about whatever toileting customs pegasi and unicorns might have, and how incompatible they might intially have been with Maretime Bay.

Come to think of it, we haven't seen a single diaper in the entire g5 or a mere reference to them so it's possible that diapers have become a lost technology from ancient Equestria. XD

Seriously though, I think it's fun to imagine under what circumstances all those characters were potty trained:

PersonallyI think that Sprout he may be potty trained after his toddler days, being a mama's boy it is very possible that Phyllis spoiled him a lot. So, if he was reluctant to give up diapers, his mother didn't put much pressure on him. Furthermore, by wearing diapers would receive more pampering from his mother through a diaper change than simply sitting in a potty, so it is likely that he deliberately preferred it that way. Possibly he did not want to give up diapers until he began to receive some teasing from his classmates and friends. and even then I can still imagine him as an adult still wearing diapers and being treated like a baby by Phyllis behind closed doors, he's a complete mommy's baby after all.

Regarding Hitch, I have the suspicion that he took much less longer than the rest of the foals to be potty trained. As you say, he is someone who always made a great effort to be a model citizen, following all the rules and guidelines. On the other hand, it could also have been the opposite case, it is assumed that he grew up under the care of his Grandma Figgy who, as we know, even in that adulthood treated him like a child in a loving and affectionate way. It is possible that under her care, he felt very comfortable and therefore he don't feel rushed to mature and grow up. In fact, I can imagine Hitch wearing diapers to this day at night and in privacy for precisely that reason, even though he obviously eventually became potty trained.

Regarding Sunny, I imagine that she may have started her potty trained on time, but in a slow way Argyle would surely have enjoyed his daughter's potty training as a special moment in his life and didn't feel too rushed to have it finished as soon as possible. So I can imagine that Sunny graduate from diapers very quickly but on the other hand, she may took a little bit longer to get out from Pull Ups.

Regarding Zipp and Pipp, I can totally see Zipp taking longer than Pipp on be potty trained because she may wanted to do it completely on her own, since she showed more Independence than her sister.

Regarding Izzy I can imagine that she was able to graduate from diapers at a young age. It is heavily implied that she was raised by Alphabittle and he should already have the experience to Potty Training a foal with Misty. Yes, that's precisely why I suppose that with Misty's case it would have taken the same amount of time or a little longer.