People usually will call the person "birthday boy/girl" even for adults.

US-ian here. Yes, "Birthday boy," "Birthday girl," nothing else. Unlike other contexts, the use of "boy" and "girl" is not seen as belittling, because a birthday party itself everyone is recapturing feelings of childhood.

In German they say, "das Geburtstagskind". Literally: the birthday child :)

Yes. 'Birthday girl' and 'Birthday boy'. And in this situation, it doesn't matter how old the person is - your 90-year-old grandmother is still 'the birthday girl' on her special day.

In English protagonist is really just used for fiction. You can talk about the protagonist of a novel or film but it's not used for talking about people at an event.