@Kari38 RE: I went ahead and did that, but it doesn't change the years in service.
Whatever your screenshot is of, it probably doesn't use the start date, but instead the first sale date. Which, naturally, is wrong thinking, because it isn't reasonable to expect and entire history of customer activity to actually be in your QuickBooks company file. QuickBooks own built-in condense feature will even delete older data you did enter.
What happens if you, as an experiment, create a sale for the customer that is back-dated way back to when they actually started? Does that change the number of years? If so, then what if you void that sale? Do the years change back? If not, then that's one way to trick QuickBooks into doing the right thing, though you shouldn't have to...
