Note the recurring three motif ..three party hats , three celebrants, three presents , three plates ( birthday cake for the eating of )..three items in the pictures on the wall etc..
It is still very much a triumvirate,( despite Eric being in DC more than in mountainview )..and this is their celebration...
It is also their first teenage birthday..their "childhood" phase is over...inspite of the "retro" innocent, even wistful image that the studied simplicity of the album photo is meant on one level to engender..
Google know more than most do, about symbolism and the psychology and perception of colour, form and image.
Every picture tells a story..this one speaks volumes.