She declared that she had a boy friend.Did I use the word''declare'' No, it's unnatural to say that. You would say: "She told everyone she had a boyfriend" or "She said she had a boyfriend". More formally, you could say, "She announced she had a boyfriend" A possibility also is "She admitted she had a boyfriend", but the meaning is different here. Here you are saying she was keeping her boyfriend a secret, but someone found out and she was forced to say she had a boyfriend.

Sure. It is fine. A declaration is a statement, usually of some importance. . So, by saying "declared" you are saying that she said it as if it was a greeat event, or important. As such, this is something that teens could easily do. . told everyone - mild Announced - more impressive a statement declared - usually even more of a statement

I think it's okay to say so if she were a princess

Announced can be used in place of declared