This differences between “å bli” og “å være” is a bit different from the “to be” and “to become” in english, but the general guidelines are this;

“Bli” involves a change from one state to another
“Være” is a fixed state

So for example:

Han blir fem år i dag. (Change from four)
Han er fem år. (Stable at five years)

De ble glade for gavene. (Change from normal to happy)
De var glade hele sommeren. (Stable at happy)

Han ble født i 1982. (Change from “not born” to born)
Han er 28 år gammel. (Stable at 28)

Hope this helps!! 😀