Error-Input string was not in a correct format

Can you please check colum[13]?

1. What id data type of that column.
2. If it is decimal then no need to convert into string 
3. you can direct use leadHeader.ChildAge = Convert.ToInt32(colum[13]);

 what values you are getting int colum[13].

If you are getting 8.4 in colum[13], then above code is correct only and it will return 8.