Subtract takes a TimeSpan and not a DateTime.

To calculate someones age correctly try something like this:

@{
  var birthDate = memb.getProperty("BirthDate").Value.ToDateTime;
        // cache the current time
        DateTime now = DateTime.Today; // today is fine, don't need the timestamp from now
        // get the difference in years
        int years = now.Year - birthDate.Year;
        // subtract another year if we're before the
        // birth day in the current year
        if (now.Month < birthDate.Month || (now.Month == birthDate.Month && now.Day < birthDate.Day))
        {
            --years;
        }
}

<span>Age: @years</span>

Remember that you can make this reusable by creating a function out of this and putting it in a @functions block:

@functions{ 

        public static int GetAgeFromBirthday(string birthDay) 
        {
       var birthDate = birthDay.ToDateTime;
           DateTime now = DateTime.Today; // today is fine, don't need the timestamp from now
           int years = now.Year - birthDate.Year;

           if (now.Month < birthDate.Month || (now.Month == birthDate.Month && now.Day < birthDate.Day))
               --years;

           return years;
        }
}

<span>Age: @GetAgeFromBirthday(memb.getProperty("BirthDate").Value)