FWIW my son's SB5 testing results included age equivalents, albeit with lots of caveats about how misleading they can be. I agree that they don't seem very useful in assessing intellectual rareness or level. One of my son's measures supposedly showed that he was equivalent to an average person 30 years, 0 months old, which of course seems questionable and doesn't help explain how to raise or educate him.* I think my tester might have included the equivalents for shock value, to prod school admins who might be in denial, and nothing more. (ETA:
This page contains a link to an overview paper on change-sensitive scoring by the author of the SB5, Gale Roid.)
* I can imagine the conversations now:
Why can't I stay out all night?
While you live in my house, under my rules, ... (etc.)
You don't control me. I'm a man.
You're 10.
When I was five, I was already the equivalent of a 30 year old man (waving a sheaf of papers). I should have the rights of a 60 year old. If you need help with the math, give me a holler.
