If you are using the Business edition, you can send your mailings to a selected group of your subscribers. It's as easy as creating a new mailing (see Creating New Message), and configuring the filter (after you click the "To" button to select the target mailing lists):
Select the lists you want to send to and configure the filters. For example, here's how you could set them up to send to all customers from New York, who were born before August 11, 1993 (i.e. are at least 18 years old, assuming today is August 11, 2011) and who want to receive special offers:
Please note that the required date format is YYYY-MM-DD (examples: 1981-08-11, 1977-12-10, 2011-05-01) both in subscriber fields and while setting up filters.
Hint: It's a good idea to preview the mailing to make sure it's going to the intended recipients (see Previewing Before Delivery to Subscribers).