The closest I can think of with my limited knowledge is Zola Budd. Set a gallon of junior records at events from 800m to 10000m, a couple of which were world junior records that still stand 4 decades later, and is now a successful ultramarathon runner in her fifties who has broken age group records. I think several years back she also ran a one off 3000m or something at the American college she now coaches at and it broke the college record (I'm not sure about that, but it was extremely fast for a near 50 year old).
I suspected Anthony Whiteman might have broken some meaningful junior record, but it seems he went into athletics quite late as his 800m pb at age 21 was 1:52. Given his destruction of master's records, I wonder just how good he could have been back in the 90's if he had started earlier? (although he would have been up against the EPO hordes).
Steve Ovett still holds the junior record for 400m for the county of Sussex (a population of nearly 2 million) AND the Sussex senior 10,000m record (28:14 in a road race in the middle of the 83/84 winter when he preparing to attempt to defend his 800m Olympic title).