If you look at the questionnaire you will see that birth date is collection in 4 columns, first two are month and second two years. So 497 would be month 04 and year 97.

Please pay attention to the questionnaires for each variable of interest. You may find in other cases (e.g. age of child at the moment) the question was asked age in years and months. So 1606 would 16 years and 6 months. Leading 0s are dropped, trailing are not.

Also 88 reflects missing value for months and 18 is missing value for calendar years.