>I'd argue his lack of history with the human village implies he's somewhat young for a youkai.
if I remember correctly, he was under the guidance of Marisa’s father for some time before he moved out due to him feeling that his ability can get better use and started Kourindou. In CoLA, his time there is rarely ever mentioned, but I don’t think it’s because he didn’t have much interactions with them. Even then, there is a lack of information on what he was like during his time in the village, so I don’t think this can be used as an argument
>He has been to the outside world also, but didn’t recognize it when he returned.
Speaking of which, there is also the fact that he lived before he came in to Gensokyo, so his age might’ve been older, also this point would have only been valid had he recognized the outside world, but he doesn’t, so that means the outside world he knows is a lot different from the one he was shown when he got there for a second time, and thus, would make the point that more than less time have passed.
>I get the feeling he's either less than a hundred years old, or just over a hundred. He seems really naive and uninformed about a lot of stuff, and quite innocent besides.
I think he’s about 150~170 ish, given the point above, and his naivety about a lot of stuff in this case might just be a result of his silent demeanor, being generally uninformed about the items that lies inside his shop, and Zun being Zun.