Michael,
Two options.
1. Deleting via database. I suggest finding a developer who can write a query which could have pulled users of certain age and delete them.
Deleting user manually via database is not recommended, since once you delete needed users from ow_base_user table, you will then have to go through ALL other tables and delete content(photos, links, newsfeed items, blogs and etc) related to those users.
2. Custom code modification.
Adding new search criteria to "Browse Users" page in admin panel. Right now you can search by real name and email. If you add a code that could have searched by birthdate, you can then easily delete users from admin panel using already existing "Delete" feature.
This option is better in my opinion, since when you delete users from admin panel ALL data related to this user is deleted from database tables automatically and you will not have to go and delete blogs, photos and etc. separately.
I suggest creating a topic under "Custom Code Modifications" section of our forum. Daisy will be able able to tell which file needs to be modified, but most likely you will have to work on creating needed code yourself. This is not a complex custom code modification but it still requires .php knowledge. So if you are not a developer, I suggest finding one.