"Boyhood" was filmed over 12 years so the aging of the characters is authentic and pop culture (mention of popular songs, books, presidential elections, etc) is inserted into the scenes as sort of time capsules.
We first meet Mason (Ellar Coltrane) at the age of six, along with his sister and single mom, Olivia (Patricia Arquette). Throughout his youth, the family moves to Texas and their dad (Ethan Hawke) becomes more active in their life.
Olivia remarries but her husband becomes abusive and they must abandon their home (and his kids that Mason and his sister have bonded with).
They move to a new area and Olivia begins teaching at a local college where she meets a young professor who eventually marries her. This relationship falls through, too, when he becomes mildly hotheaded towards her.
As a teenager, Mason gets a girlfriend and begins using marijuana and alcohol like most kids his age. His dad gets remarried and the kids meet her hyper-conservative parents.
As a senior in high school, Mason develops a love of photography, a part-time job, and a new girlfriend. A year later, all of his family and friends gather to celebrate him graduating high school. His mom cries knowing both of her kids are now grown up and she'll be alone.
The film ends with Mason starting college and hiking with new friends.