I've come across variations of this old wives' tales before and never seen any any legitimate justification for it other than hand-waving about acids combining in some "harmful" manner.

(I've been eating persimmons for many years and never experienced any adverse reactions, even though I'm pretty sure I've had milk/yoghurt/dairy products alongside it. And coincidentally, I gave my 11-month-old a lunch couple of hours back that contained yoghurt and followed that up with a persimmon and haven't notuced anything unusual.)