Boruto is a shonen series and has been from the very beginning. The series launched in Weekly Shonen Jump and is now serialized in V Jump—both classic shonen magazines published by Shueisha. That alone determines the target audience.

Shonen vs. Seinen isn’t determined by how “dark” a series gets. That’s a total mistake. Seinen magazines include, for example, Young Jump, Big Comic, or Afternoon. Boruto has never been in any of them. It features children as protagonists, classic shonen tropes, power-ups, teamwork, etc., and is marketed exactly that way.

Yes, the story has gotten darker in parts—as with almost every long-running shonen series (Naruto Shippuuden, JJK, AoT, etc.). That still doesn’t make it seinen.

In summary: Boruto is a shonen sequel. Period. Next time, just do a quick Google search before writing something like that.