First, the birthdate have to be converted to datetime type using str_to_date function in mysql to convert from varchar to datetime, see document
here.
Secondly, there is really no need to store age in database as it is a moving number that is derivable and is best derived from birthdate in code.
Thirdly, better to change all the date fields, eg. birthdate, dateofemp etc, to datetime data type. You should be able to find examples on the web.