Are you referring to the Birthday calendar, or to a birthday entry in your regular calendar? The Birthday calendar does not have any settings for notifications. They are considered All Day events. If you are talking about birthdays you create in your calendar program, that is different. My Birthdays are created from Outlook information for contacts and cannot be modified in any manner, including alerts. Unless you are referring to default alerts under Settings>Mail, Contacts, Calendars>Default Alert Times.
First of all thanks for the fast reply!
I'm talking about the birthdays created from my iCloud contacts. They show up in my calendar with this red birthday symbol. Under general settings, I can choose default ALERT times for Birthdays, Events, All-Day Events. I only switched on Birthdays for 9:00, so I assume that I should get an alert at that time, and I used to get them before upgrading to iOS 7 (although I'm not sure I was able to choose the time)!