In season 1, the core Stranger Things kids were mostly around 12–14 years old in real life, playing characters who were about 12.

Ages of the main kids in Season 1

  • Millie Bobby Brown (Eleven)
    • Character age in season 1: about 12.
* Real age while filming/airing season 1: around 12.
  • Finn Wolfhard (Mike Wheeler)
    • Character age in season 1: about 12.
* Real age during season 1: around 13.
  • Gaten Matarazzo (Dustin Henderson)
    • Character age in season 1: about 12.
* Real age during season 1: about 13–14, slightly older than Dustin.
  • Caleb McLaughlin (Lucas Sinclair)
    • Character age in season 1: about 12.
* Real age during season 1: about 14.
  • Noah Schnapp (Will Byers)
    • Character age in season 1: about 12.
* Real age during season 1: about 11–12, the youngest of the core kid cast.
  • Natalia Dyer (Nancy Wheeler)
    • Character age in season 1: about 15–16.
* Real age during season 1: about 21.
  • Charlie Heaton (Jonathan Byers)
    • Character age in season 1: about 16.
* Real age during season 1: about 22.

Overall, the “kids” were early teens playing 12‑year‑olds, while the teen characters like Nancy and Jonathan were played by actors in their early twenties.

TL;DR: The main party (Eleven, Mike, Dustin, Lucas, Will) are around 12 in the story, and the actors were mostly 12–14 when they first filmed Stranger Things season 1.

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