Millie Bobby Brown (who plays Eleven) was eleven years old during casting and filming.
Friends don't lie.
In the early 1980s, the lights on the kids' bicycles would have been powered by a tire-driven dynamo. The dynamo's output and thus the light level would have varied according to the bike's speed, becoming totally dark when the bike was stopped.
Additionally, as with the "flashlights" anachronism, the headlights would have been yellowish, not the blue-white of a modern LED headlamp.
The Stranger Things opening titles and fonts mimic the film grain and look of 1980's television series opening credits.