Yeah we do, because their 'first year' isn't their age. We do their age in (also zero-indexed) months/weeks/days. In Indian English terms, we do 'complete' age

Yeah we do, because their 'first year' isn't their age. We do their age in (also zero-indexed) months/weeks/days. In Indian English terms, we do 'complete' age - aiui more common in India is to one-index, i.e. you're the age of the year you're in, and to disambiguate you might hear someone say they're '35 complete', meaning they have had 35 anniversaries of their birth (36 incomplete).