Complex sentence: At what age old age begins cannot be universally defined because it differs according to the context most developed world countries have accepted the chronological age of 65 years as a definition of'elderly' or older person.
Split into simpler sentences: At what age old age begins cannot be universally defined because it differs according to the context. Most developed world countries have accepted the chronological age of 65 years as a definition of'elderly' or older person.