Hovering over a PM Community user profile shows the Registration Date.
PM Community accounts are separate things from PM Self-Serve accounts - anyone can join the community without being a customer - so they're not strictly correlated. In fact, inactive PM Self-Serve accounts will eventually expire and terminate while inactive PM Community accounts appear to stick around forever, so it's possible for "old" Community members to have "new" Loyalty counts.
But they are loosely correlated. At least they tend to be for most Community users. Because many people join the community to ask questions before signing up as new customers (within a month or so). Because many more become customers then create a linked community account to set up their potential Community Rewards (usually on the same day).
So Registration Date is a fair indicator of how long a PM Community member has been a PM customer. It might be wrong, it might be corrected by the user volunteering more detail, but it's usually close enough.
