when does a disadvantage become a disability

eg not being able to walk quickly is a disadvantage. not being able to walk at all would be a disability, right?

wearing glasses = disadvantage
being blind = disability

is it the point at which it interferes with ur every day life?

i am puzzled that deaf/Deaf ppl say they don't have a disability
if u cannot hear at all surely that is a disability
not bc society is biased (altho it is_ but bc of the things it stops u doing