#he respected Kuwabara enough in the Maze castle because of Kuwa's relation to Yusuke but didn't change his habit of underestimating others

[jilted-love]

I personally headcanon Hiei to be in his 20s to 50s because it's canon that he neglected looking for Hyouga after he felt more confident and immersed in his own power and in my own interpretation, that's a good chunk of years of delay. Why? Because regardless of canon, I want Hiei to grow in power and reputation in Makai and become great enough that he attracted Mukuro's attention before he even left.

Hiei had a lot of achievements in Makai which he gave up to come looking for Hyouga again. He also got a lot of enemies that could have targeted him while he was weakened and only decades of reputation in Makai earns him all that.

This will also mean that he neglected Yukina before he looked for her, before he knew of her existence, which made Yukina suffer longer in captivity and will give Hiei more guilt and reason not to admit to being her brother. Hiei was too murderous and loved being in power that he let his sister suffer longer when he could have saved her faster if started looking much earlier. He was punishing himself for losing his stone when he accepted the pain of jagan operation but it could also be punishment for the delay it took him before he cared enough to look.

If Hiei comes off as naive, that's because he overestimates his own power and looks down on his opponents which is consistent with him since childhood, since meeting Yusuke and then meeting Kaito. Yukina on the other hand can be naive due to choosing to view people as innocent until proven guilty which is not the way of life around crime in both worlds. Their age can be blamed for that naivety but young age for youkai doesn't mean teenage years because they're not human. Koorime are a long lived race. They could be in their 50s to 90s and still be considered young.