well, you can of course do whatever you want

, but I don't think that is what you are really asking.
Is it permitted under the regs?
yes, but...
1. when testing a plan you are supposed to use normal retirement age under the plan, so you can't simply decide to use that as your testing age.
(see 1.401(a)(4)-12 defintion of testing age)
2. My understanding is that you could write a plan to contain language that for a(4) testing it would be SSRA - at that point it is also my understanding you would now have to check BRF , e.g. ees at SSRA 65, ees at SSRA 66, ees at SSRA67. chances are the plan would fail at that point.
if your HCE is born in the first half of the plan year, using age definition nearest sometimes helps, especially if the NHCEs were born in the second half of the year.