How can I change the customer start date?

You can change the Start Date on the customer's Job Info tab, where the Start Date field is displayed.

Thank you for your response. I went ahead and did that, but it doesn't change the years in service. 

Hi there, Kari38.

Can you take a screenshot about the years of service? I'll keep this post while waiting for your response. Thank you and have a great day ahead!

This is a screenshot from the company snapshot page. This company has been with us since 3/24/1986, but it still only shows 21 years old.

@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...