There are several ways to filter your participants and create groups as described in the article on Creating a group of participants using filters.  
In the same way that administrators can filter by any other signup form question, with the year of birth option, admins can now filter by year with "On and after" or a "Before" filter by clicking on +Filter > Sign up questions.

Enter the dates as required and click Apply Filters. This will show you a list of participants born within that date range or a selected date.

Selecting the same year in both fields acts as a filter for that year alone

Contact our support team if you need any further assistance via chat or email support@engagementhq.com