In the year of Space Age 216, beyond the limits of the Milky Way, the sonar of exploratory vessel "Kikumasa" picks up an unconfirmed energy signal. Excursion pod #18 is dispatched to the location.

Why do people think that everyone who watches anime is a kid?

Animation in general is an art form, but it’s very common worldwide to treat a person who watches animated shows instead of live action shows as a kid or nerd, even in Japan. On the West, the Japanese animated series Pokemon was on the peak of the popularity in the 1990’s, and it’s a kids series.
Since many people were used to Pokemon, Dragon Ball Z, Speed Racer, or Naruto in their childhood/teenagehood, this can fuel this association.
Also, before the word “anime” there used to be the word “Japanimation”. In Japan, people usually associate the word “anime” with animation in general, rather than just Japanese works.
In fact, “anime” means “animated cartoon” in Japanese.
Shows like Sazae-san, Anpanman, Doraemon, PreCure series, Sanrio’s shows are very popular there amongst children and family demographic.
Talking about “nerd” side, there is teen/adult animation in both countries.
America has Marvel and DC, Japan has Gainax, Studio Pierrot, Square Enix etc. All that comes to the point that animation and live action movies are produced differently.
Animation is usually made by drawing on cel, taking pictures of characters and moving them in different poses, or 3D software.
Live action is only captured live on the camera, although some alterations with the computer editing exist.
Live action is more realistic because it captures the real live scenes as they are, while animation only incorporates it by using motion capture and other technologies. Since a person who accepts reality as it is is generally treated as an “adult”, and a person who tries to stick in imaginary realm (even if it is possible to replicate it with cosplay, decorations, styling etc) is considered as a “child”, that is why animation as an art in general can be perceived as “childish” by people.