Why do I get error when replacing datagridview values in vb?

I have a datagridview which holds an imported excel file. I also have a MySQL database which holds the values i want to replace in my datagridview and the values that will replace the previous values.

This is my database:

What I want to accomplish is to loop inside the datagridview cells and find the values inside the Content field and replace it with the corresponding value in the Convert field. For example all 1 will be replaced by 11, all Name 1 will be replaced by Std Name 1, all 18 will be replaced by 18 Years Old.

I use the following code:

MysqlQuery = "SELECT * FROM contents"
    MysqlComm = New MySqlCommand(MysqlQuery, MysqlConn)
    MysqlReader = MysqlComm.ExecuteReader

    While MysqlReader.Read
        Dim content = MysqlReader.GetString("Content")
        Dim convert = MysqlReader.GetString("Convert")
        For Each row As DataGridViewRow In dgvFile.Rows
            For Each cell As DataGridViewCell In row.Cells
                If cell.Value IsNot Nothing Then
                    If cell.Value.ToString = content.ToString Then
                        cell.Value = convert.ToString
                    End If
                End If
            Next
        Next
    End While

        MessageBox.Show("File successfully converted")

This code converts 1 = 11 and Name 1 = Std Name 1 but not 18 = 18 Years old and it shows this error.

QUESTION Why do I get this error and how can I fix this? Answers and suggestions are well appreciated. Thanks. :)

UPDATE For those who can't see the error message, here it is.

The following exception occurred in the DataGridView:

System.Exception: 18 Years Old is not a valid value for Double, --->
System.FormatException: Input string was not in a correct format.
 at System.Number.PraseDouble(String value, NumberStyles options,
NumberFormatInfo numfmt)
 at System.Double.Parse(String s, NumberStyles style, IFormatPRovider     provider)
 at System.ComponentModel.DoubleConverter.FromString(String value,   NumberFormatInfo formatInfo)
 at     System.ComponentModel.BaseNumberConverter.ConvertFrom(ITypeDescriptorContext  context, CultureInfo culture, Object value)
--- End of inner exception stack trace ---
 at    System.ComponentModel.BaseNumberConverter.ConvertFrom(ITypeDescriptorContext   context, CultureInfo culture, Object value)
 at System.ComponentModel.TypeConverter.ConvertFrom(Object value)
 at  System.Windows.Forms.DataGridView.DataGridViewDataConnection.PushValue(Int32  boundColumnIndex, Int32 columnIndex, Int32 rowIndex, Object value)

To replace this default dialog please handle the DataError event