Bob says he had a birthday cake on his 18th birthday.

Bob said he had a birthday cake on his 18th birthday. It is a reported speech.

But he could say it right now and narrative his past?

Yes, that's possible.

You are on the phone with brotherBob and you're also talking to your mother. Mom asks you to ask him if he had that amazing chocolate cake with the strawberry/banana cream filling at his 21stbirthday. You ask him. He says that he had that cake at his 18th birthday and not his 21st, because he was dating Monica then and remembers how much she liked it. You look up fr

Bellyhe could say it right now

It doesn't matter if he said it right now or last week. The only thing that is important is the emotion of the current speaker. If that speaker feels what he is reporting is current (or wishes to project it that way) then he may use the present tense. Also the present tense is appropriate if the speaker thinks of it as a habitual

HuevosAlso the present tense is appropriate if the speaker thinks of it as a habitual action

Good point.

Bob says this every time you ask him, even though we all remember that it was his 21st birthday, and not his 18th.