Ancient Vs Old

'Ancient' is the opposite of 'modern' whereas 'old' is the opposite of 'young'. Moreover 'ancient' refers to very old things. And 'old' can at times be used in the case of a very young child as well. As for example,

• The ancient Rome was the cradle of civilization.
• The ancient Mariner narrated his story of adventure.
• Death overtakes all alike, the young and the old.
• My baby is only two months old.