I think people's main qualm is the maturity difference cause like, Pony is 14 and Johnny is 16, so from a modern, average person standpoint it's like: "the average 14 year old is in middle school or freshman year of hs and a 16 year old is freshman/sophomore age!" and i do understand that, like I've been 16 and I had a 14 year old friend who had a crush on me and I would never date them in a million years. They're just too young for me, and we have a huge gap in maturity despite us being pretty similar people. Even just being friends is difficult because I've gotten older and matured, and they're still. yknow. 14. ive also known 14 year olds who dated 16 year olds, and 15 year olds who dated 17 year olds, and I cautioned them because the average person (average being keyword) wouldn't be in highschool, dating freshman or people who are barely out of middle school. It's more likely that they're predatory or immature themselves, and in each case where I warned the younger person I ended up being right - the older person was either severely immature, or creepy. Not to mention that while as little little kids and adults, 2 years is nothing, but as tween and teens 2 years is a lot. You're in the formative years of your life, and 2 years isn't just 2 years - it's two years of growing, life experience, forming your sense of self, developing your opinions and morals, your body changing, etc.
The thing with pb&j though is 1) they are fictional. enough said. 2) the time the outsiders is set in. It's the 60's. 2 years in a friendship or relationship is nothing. Hell, 5, 6, 7, 8 years isn't much! 3) remember how i kept emphasizing average person/people above? Yeah, the average, financially stable, mentally stable, 14 year old wouldn't be mature enough for a 16 year old, and the average, financially stable, mentally stable 16 year old would be in a different place in their life and probably shouldn't date a 14 year old. I've worked with 14 year olds, i've been friends with them, i've been around them, and i personally see them as kids despite not being much older than them. But pb&j aren't the average characters. In the time and setting they're in, plus their long friendship, they know each other better than they know themselves, are in the same year of school, and have a smaller maturity gap.
I could hypothesize about how people who are mentally ill, mature for their age (which is a saying i hate but the only way of phrasing this i could think of), grew quicker, and experienced more, seem to see the world differently than people who had a more 'normal' or 'healthy' or 'acceptable' upbringing, due to a difference in experiences and what they consider normal, and therefore people will have different expectations for what is considered a healthy age gap, but I've written a lot more than i expected.
Anyways!! These are just my thoughts on the matter! I have no clue if I'm actually right about any of this, I'm not a child psychologist, or sociologist, or any of that, and personally I really like PB&J! In the end they're fictional, so I personally don't care about the age gap. I just have a lot of curiosity and thoughts on the way different people see certain age gaps so i wanted to ramble :3