I know it’s arbitrary, but my Xbox account has been linked since 2013 and my PS account has been linked since 2016, and the account only displays the PS Link time instead of the Xbox. I’ve tried going over to the Xbox account and it still says 3 years even though that account has been linked since 2013. If I can unlink the Xbox and PS account and get the right number displayed then I’d love to know how.
The profile age depicts when your Bungie.net profile was created, not when your Xbox or PlayStation account was created. There is no way to change or alter your Bungie profile age, it’s just a slow crawl to infinite.