In "Weathering with You", what was Hina's real age? If I recall correctly, when Hodaka asked about Hina's age, Hina responded that she was 


   almost 18 years old.


But when Hodaka got arrested, the detective said that Hina was actually 


   15 years old, which means he's younger than Hodaka.


But the Wiki says that Hina was 


   18 years old. 


Did the police gave incorrect information, or is the Wiki incorrect, or any explanation to the discrepancy?
From what I could understand, the wiki is not incorrect. You are misled by the fact that you isolated this sentence out of the whole article. The wiki provides the timeline where the events you cited (when Hodaka speaks to the detective) happened on August 2021. Hina was 15 years old at this time. During their altercation with the police, it would make sense that the police would say she's 15 since she is at that time. The wiki page you cited as incorrect states there that Hina's age is 18 as of 2024, which is 3 years since 2021.

Thus, the wiki page is just putting the latest age information from the movie timeline. It is not incorrect, the wiki just prefers to put the latest information depending on where the story has progressed or ended.

Both information from each websites are correct. You just need to understand that the other website you linked recounted the events on August 2021 (Hina is 15 years old at this time).