Siri miscalculates ages
Although you probably know your own age, Siri doesn't calculate how old they are for your contacts either. The flaw is in the fact that it doesn't count when your birthday is. If you turn 30 this year, but aren't yet, Siri will calculate that you are already 30 years old.
Do you want to know how old a contact is? you can first ask Siri: “When is my birthday?” or “When is Pieter's birthday?” and you will get a correct answer. That way you at least know whether the birthday has already passed this year. After that you can possibly ask “How old am I?” et cetera.
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It is likely that this bug can be resolved remotely by Apple. So you probably don't have to wait for a software update. There is no detour, except adjusting your birthday to a date that has already passed this year. This error is sloppy, because this is certainly not the first time Siri has made a mistake with fairly simple requests.
For example, it has happened several times that Siri reveals announcements or event dates. It was announced that Siri would also come to the Mac. It was also announced early on that Siri speaks Japanese. Birthdays didn't always go well either, because years ago Siri sometimes said wrong when a birthday would take place.