My personal bests became my lifetime bests the moment I ran them.
This is the correct answer. Maybe the OP is asking when we KNEW that our PRs were our lifetime bests?
I ran my last PR (marathon) at age 27. My 10K and under PRs were set in college. I didn't know until my 30s that this was the case. I held out hope for several years, believing if I just tried this or that...