Three years is still the age range for 'Romeo and Juliet' laws, if they're a thing in a jurisdiction. In other words, depending on where you are, it's technically legal.
And, yeah, the fact that this is a flashback to why he started wanting to see movies with Shia was poorly framed. It finally clicked once we got to the part about everybody wanting to see a movie and Tasuku kept giving push-back, because if felt so out-of-character from how he's been up until now. Like, why wouldn't he finally just go 'yeah, let's watch' the moment Shia said she wanted to see it?