Yes, the Maximum age field is to define the retention period for this threat source, defined in relative time.
The default maximum age is -30d for 30 days of retention in the KV Store. To remove the data more often, use a smaller number such as -7d for one week of retention. The maximum age field cannot be left black because storing the collection indefinitely can impact performance.
Basically it removes threat intelligence from the KV Store collections in Splunk Enterprise Security based on the date that the threat intelligence was added to Enterprise Security.