Why are the new Star Wars busts 18+?
Product description (cannot embed URL for description, so you have to click on "DESCRIPTION" tab) explains why adults are a target audience. (emphasis is mine)


  It’s a super LEGO Star Wars™ gift idea for adult fans and offers a
  testing challenge for experienced LEGO builders, with the familiar
  shape and authentic details of the helmet beautifully recreated using
  LEGO bricks.
  
  Pay homage to one of the most fearsome bounty hunters in the Star
  Wars™ universe and challenge your LEGO® building skills to construct
  this awesome Boba Fett Helmet (75277) memorabilia model for display.
  
  The authentic details of the Boba Fett Helmet are beautifully
  reproduced with LEGO® bricks. Displayed on its base with nameplate,
  this model will spark memories of thrilling Star Wars™ scenes.
  
  This Boba Fett Helmet is part of a range of collectible LEGO® Star
  Wars™ build-and-display models – also watch out for the Stormtrooper
  Helmet (75276).
  
  The best birthday gift, Christmas present or special surprise for Star
  Wars™ fans aged 18+ and any experienced LEGO® builder, this 625-piece
  Star Wars Boba Fett Helmet offers a challenging construction project.
  
  This LEGO® Star Wars™ Boba Fett Helmet measures 8.5” (21cm) high, 4.5”
  (11cm) wide and 4.5” (11cm) deep – it doesn’t take up lot of space,
  but its striking details are sure to delight any Star Wars fan.
  
  This cool, battery-free construction set for creative adults offers a
  stress-relieving, relaxing build. So, take time out, find your
  building Zen and construct a fantastic Star Wars™ character
  build-and-display model!
  
  Thinking of buying this Boba Fett memorabilia for a Star Wars™ fan new
  to LEGO® sets? No worries. It comes with clear, step-by-step
  instructions so they can take on this complex building challenge with
  confidence.
  
  LEGO® Star Wars™ construction sets are great for kids and adult fans
  to build and display the models, recreate classic Star Wars movie
  scenes and create their own exciting stories. There’s something for
  everyone!
  
  No need to use the Force to connect or pull apart LEGO® bricks! They
  meet the highest industry standards to ensure consistency and a
  perfect, easy connection.
  
  LEGO® bricks and pieces are dropped, heated, crushed, twisted and
  analyzed to ensure that this LEGO Star Wars™ set meets the highest
  safety standards on Earth (and in galaxies far, far away!).


Basically a little more challenging build and not intended for play are the main reasons, I suppose.

Although I'm not sure why they both mention "+18", "adults" as well kids in the same description, like here:


  LEGO® Star Wars™ construction sets are great for kids and adult fans
  to build and display the models, recreate classic Star Wars movie
  scenes and create their own exciting stories. There’s something for
  everyone!


EDIT. LEGO has released new 10273 Haunted House set with a press release explaining why some new sets have "18+" marking.


  Addressing the new packaging, Senior Marketing Manager Anders Hellegaard Iversen said: “So many of our older builders love the Creator Expert products, as they allow them to show off their passion for the bricks. Whether it’s their favourite film moments, sport icons, travel destinations, buildings, fairgrounds or vehicles, LEGO sets help to bring these to life in brick form. That’s also why we have decided to stop using the Creator Expert branding and, instead, use the new adult-focused packaging design to make it clearer which hobbies or brand products they can relate to. We hope this will make it easier for fans to track down models they would be proud to display once completed. Of course, our product range will continue to include the same advanced-build, collectible products that guarantee hours of building enjoyment for fans.”