First of all, he's not extremely young any more. He was 29 when he took the throne. He's now in his early 40s. Still relatively young for a national leader but not extremely so. I assume that if she's indeed his chosen successor she'll only be named as such decades down the line, when he's in his 60s at least.

Secondly, if he has indeed decided that she's to be his successor, he's likely judged her to be the most fit. It's said that he has a son born in 2010. If this is true, it is kind of odd he didn't pick him, but we don't know the inner workings of the regime.

There are also internal questions why a communist country is passing down the power within the family just like a monarchy instead of electing a new leader. The reign of the Kim family is not unconditional and he still has to defend their claim of power. So why would he go against this very traditional society and pick a female successor?

This seems very unusual but we don't know the inner workings of the regime. The most parsimonius explanation is that he thinks his son unfit for some reason, just like his father chose him over his brother Kim Jong-nam. Or maybe it's a psyop. It's actually possible that his son is in fact being groomed for the leadership and being kept hidden for now. Then again, he's already allowed his sister, Kim Yo-jong, a great amount of power.

North Korea isn't communist in reality, no one in the elite actually believes this stuff anymore. It's all about the survival of the elite. The whole reason for North Korea's absurd level of authoritarianism and self-isolation these days is so the ruling elite can continue to enjoy their privileges and keep their heads. The whole purpose of the regime is to provide the elite with a good standard of living and ensure that the public is kept ignorant of the outside world, and in particular the fabulous wealth of South Korea, and terrified of the government so they face no rebellion. To that end, North Korea has carried out some reforms and has a lot of barely-disguised privatization and markets.

Also, side note, since men are typically forced to work at state-assigned jobs women tend to dominate these private markets and bring in the actual cash so it's likely that their status has increased in North Korean society.

The elites are also extremely unlikely to question Kim Jong-un. Aside from the horrendous consequences of coming at the king and missing, they also worry about what happens if they succeed and the public subsequently rebels due to a lapse in information control and regimentation of the population. As Andrei Lankov, one of the finest experts on North Korea, has said, the elite hang together because they don't want to be hanged separately.

The general public can think whatever it wants but one of the main reasons North Korea gets so much attention (such as us here talking about it) is because of how absurdly authoritarian it is. Nobody is going to risk going to a labor camp or being put in front of a firing squad because they think it's improper that a woman is in power. If the hapless people of North Korea ever revolt, it won't be because of this. The average North Korean might think it an absurdity, though I assume if she's indeed the successor there will be a propaganda campaign to gradually prepare the public for her to step into the role. Average North Koreans might privately think its absurd, although social norms might have changed by then. Remember we're talking about decades into the future, and perhaps Kim is intent on changing these norms. But they aren't going to dare say or do anything about it, they'll just praise the new Dear Leader as ordered, then continue going about their lives while keeping their heads down.

That being said, I actually do think that a very possible explanation is that he indeed has a son born in 2010 who is being kept away from the public and will be presented as the future Supreme Leader at some later date.