You can book any age ticket on any fan number. I've booked kids tickets under friends' names before and the other way round for away or cup games. All they care about is that the person entering the stadium is eligible for the ticket bought.
Unless they've changed in the last year or two, they always used to based it on the age of the person on a certain date, usually in July or August at the start of the season.
For instance for someone turning 18, as long as they were 17 at the start of the season, they would still get the u18...