Seven is not a cliff edge, but it is the year screening starts earning its keep.
Nothing happens on the birthday. What changes is the arithmetic: kidney, liver and thyroid values drift slowly, and a single reading a year is not enough to see the drift while it is still cheap to treat. From seven we move to two exams and one full panel a year, and we compare against your dog’s own numbers rather than a population range.
The three things we watch hardest are weight trend, mobility and drinking. All three are visible at home before they are visible in a blood test, which is why the questions at the start of the appointment matter more than anything we do with a stethoscope.