Well, my question actually came from an anime movie (please forgive me for asking from an anime), Makoto Shinkai's romance big-hit film Weathering With You (天気の子, Hepburn: Tenki no Ko), where at some point Hodaka Morishima started calling Nagi (Hina Amano's little brother) "Senpai". The reason for this, I presumed, is because of Nagi's experience dating with girls, and giving Hodaka advice for his relationship with Hina. However, my question is, is "Senpai" used like this? Is it normal for an older person to call a younger person "Senpai"? Especially a lot younger in this case, almost 6 years smaller. Does "Senpai" only mean that a person is more experienced in something than you, or does it also implicitly infer that that person is older than you? I'm totally not a Japanese speaker, so please forgive me if this is a very dumb question. I'll appreciate any explanation on this. Thanks in advance!
Note: I'm NOT asking why Hodaka calls Nagi "Senpai", or anything about the anime, it's just that I used the movie as an example. If I was asking about the movie, I would've asked on the Anime & Manga site instead. Here, I'm asking about the proper usage of "Senpai", as well as whether or not there's an inferred meaning in that term.