You can set up an age email notification to be sent out from the system. Please see steps below:
Click Settings, usually within Configuration or Admin Menu.
Select Notifications.
Click Add Notification and fill the below details:
Description: Name of the notification.
Type: Date.
Table: Person.
Field: Choose Date of Birth.
Click Add Email and select Email Template.
Under Send Email - Enter the specific time, e.g. 18 years after.
Select Recipients and then click Save.