Board Thread:Questions and Answers/@comment-32757117-20170803014555/@comment-24830484-20170805221249

Quirks are basically paranormal mutations. People usually start to develop them around the age of 4. Quirks tend to continue to grow along with the user. So I would assume in Tokoyami and Ashido's case they started slowly morphing into their current forms to better channel their forces. Probably giving their parents a good scare too.