Hope and Rook are 23 at the start, but turn 24 later in the first series. However near the end of the first series, Ben, Gwen, Kai and Cooper turn 18 in addition to Kevin and Ester becoming 19. Plumbers of Bellwood takes place a month after Dark Endgame, alongside taking place after four weeks after Omnipoint and also confirming that Ben, Gwen, Kai and Cooper are 17, alongside with Kevin and Ester being 18.
This takes place right after my fanfic that officially confirms that Omniverse is actually a virtual reality, titled Omnipointand my epilogue fanfic that takes place right after Ultimate Alien, Dark Endgame. (though it is similar to The X-Files) influenced series. Goodbye, X-Files influenced series, hello S.H.I.E.L.D.
Because of this, I'm officially changing the name to Ben 10: Plumbers of Bellwood. I've decided to rework my Ben 10 Horizon sequel into a whole new concept that is inspired by the popular, yet overlooked show Agents of S.H.I.E.L.D. plus the aborted sequel series concept by the creators, Ben 10 Hero Generation.