The main “Stranger Things” kids were around 11–14 years old when the show first started filming and season 1 was released.

Core ages when it started

For season 1 (released 2016, filmed 2015), the child actors were roughly:

  • Millie Bobby Brown (Eleven) – about 12 years old at debut.
  • Finn Wolfhard (Mike Wheeler) – about 13 years old at debut.
  • Noah Schnapp (Will Byers) – about 11 years old at debut.
  • Caleb McLaughlin (Lucas Sinclair) – about 14 years old at debut.
  • Gaten Matarazzo (Dustin Henderson) – about early teens (around 13–14) at debut, similar to the rest of the core group.

In the story, those characters are playing about 12‑year‑old middle‑schoolers in late 1983.

In‑universe kid ages in season 1

Within the show’s timeline:

  • The boys’ party (Mike, Will, Lucas, Dustin) and Eleven are portrayed as about 12 years old in season 1, set in November 1983.
  • Fan timeline breakdowns consistently place them as 7th‑graders around 12, turning 13 soon after.

So, put simply: the actors were roughly 11–14, and the characters were written as around 12 when Stranger Things started.

Information gathered from public forums or data available on the internet and portrayed here.