Purdah

Assalam o alaikum,

Respected Sir, I want to ask that from what age a girl should observe sharie purdah/hijab and till what age. i.e minimum age and maximum age. Should she have to make purdah till she is dead or up to say 80 years of age or 95 years of age etc. Some say that she is old so she does not need to make purdah in front of others as who is going to look at an old lady. Is this concept there in Islam. Awaiting your reply.

Jazzak Allah!