Redshift/postgres, calculate the interval in years between two days. (age function)

create function eta(date, date)
  returns int
stable
as $$
  select
    case when $1>=$2 then
    case
      when ( add_months($1 , date_diff('year', $2, $1 ) * (-12) ) ) < $2
      then date_diff('year', $2, $1 ) - 1
      else date_diff('year', $2, $1 )
    end
  else null
  end::int
$$ language sql;