Quirk Activation (for World Heroes’ Mission Reaction)

My understanding is that it varies. Some are born with their quirk already manifested while others occur in early childhood. I believe that it's by age 4. If they don't, then they are unlikely to have one. That's why Deku's mom takes him to the doctor around that age - it hadn't developed and they wanted to see if he had one. But yes, some are born with their quirk manifested. The very first quirk user was a baby who was born glowing. There's even a mention somewhere in the show that Present Mic broke the eardrums of everyone in the room during his birth. To go into Erik's concerns, not entirely sure - if Google is right, then yes - all heteromorphs (those with physical quirks to their bodies) are born that way with their quirks already manifested. You would think that would be dangerous - but perhaps the technology or just other quirks negate the dangers?? I think Ruff's right - the writer probably didn't really think it through.