The package aging parm is a number of days (set in global admin) that a package should be in BAS status (installed in production) before the meta data of that package (VSAM records stored in CMNPMAST) are allowed to be removed during ChangeMan ZMF housekeeping. The higher this number of days, the more people like IT auditors will like it (they are often the ones who are asking for such high number of days). While many countries have laws that force such data to be kept for (e.g.) 7 years, or even more ...
Typical (often used) values for this parm are 365 (1 year) or 999 which used to be the maximum number that was supported by ChangeMan ZMF until a recent release (in which it was increased to a maximum of 9999 days, somewhere around 30 years now, and which is virtually the same as do not ever archive any installed change package).
However, the higher this number, the bigger the size becomes of the ChangeMan ZMF meta data files, like CMNPMAST. And because of that, performance (eg for only queries) goes down, CPU usage goes up, and backups take longer. These are reasons why you should have a small number of days. Ideally the number of days should only like 1 or 2 months (60 days or so).
For some simulations to further illustrate this issue, consider checking out this related link: BAS packages waiting to be aged.
Reminder: if you're in a hurry (anxious) to get Dr.Chgman to deliver this item faster, consider contributing via the Z-Bounties program.