The story was about a family, over a period of 8-10 years.

The movie took that long to make.
Because the makers filmed it over that period of time.

The people aging was real.
The kids got taller etc.

Boyhood is a 2014 film about a boy's life from age 6 to 18, and it was shot over 12 years so that the actor aged along with his character.

The same director is currently working on another film that's being shot over 20 years.