If someone is born before 9 months. For example miscarriage like 6 months. And you were born after 9 months. So that person is older then you even he existed for less time.
Baby A. Born after 9 months. After 12 months will have the first birthday. Total alive 21 months, but 1 year old.
Baby B. Born after 6 months. After 12 months will have first birthday. Total alive 18 months, but 1 year old.
So if both started same time. Baby B have existed for less time, but he will be older. Baby A will be 9 months old, when baby B will be celebrating his first birthday.