I recently had a life changing revelation, and it’s about these stupid red sneakers Izuku and Tomura wear. Going all the way back to the start of the series, let’s recall something we learned in the first chapter;
Quirkless people have an extra joint in their pinky toes.
That’s how the doctor at the start knows Izuku is quirkless vs a late bloomer or someone with a quirk waiting to be triggered. People with quirks don’t have an extra joint. This joint is also something developed fairly young. As far as we know, gaining a quirk later in life wouldn’t change the status of this joint. So, if you’re born quirkless, then you’ll have that extra toe joint for the rest of your life, even if you get a quirk later.