Writer-director Richard Linklater took 12 years to make Boyhood, because he wanted to show the physical changes in Ellar Coltrane from his first day at school to his graduation from high school 12 years later. And the physical developments of Coltrane and his sister in the movie are fascinating to watch.
But I'm not sure Linklater benefits much more than that from his much-discussed gimmick. A great deal happens during the 12-year period of school, and even with over two-and-a-half hours, Linklater doesn't have much time. The very important matter of sex, for example, gets just about three short vignettes: the introduction, the waking up after the big event, and the goodbyes sometime later. Almost everything important is told to us in the third scene; very little is shown.
This is pretty realistic as to how we really experience our friends and maybe even how we remember events in our lives, but it isn't the form for drama we can feel, and Boyhood seems a little cold. It's informative, but not emotionally involving. There is a single isolated episode of bullying in school, but nothing to indicate whether bullying was an important thing in Coltrane's life-- or, the life of the character he plays: nobody has ever claimed that Boyhood is a true story. The closing credits include the usual statement about resemblance to real people being unintended and coincidental. And camerawork is clearly not improvised-- ask yourself how the matter of passing notes in class could have been photographed in a real class.
As is common nowadays, Boyhood includes a lot of good scenes and excellent acting all around. But it doesn't leave you with a feeling of having experienced very much.