Dynamic WHERE Operator Based On User Input

Let's say I have a table with 3 fields: an ID field (primary key, set as an id field, etc.), a Name field (nvarchar50), and an Age field (int). I have a form that has three elements:

DropDownList1: This drop down list contains 3 choices- "=", ">", and "<".

Age: This text box is where someone would enter a number.

Button1: This is the form's submit button.

I want someone to be able to search the database for entries where the Age is either equal to ("="), greater than (">"), or less than ("<") whatever number they enter into TextBox1.

The code-behind is shown below. The part I'm confused about is that if I load this page, the query works the -first- time. Then, if I try to change the parameters in the form and submit it, I get the following error:

"The variable name '@Age' has already been declared. Variable names must be unique within a query batch or stored procedure."

Any help would be appreciated.

Here is what I'm using in my code behind:

    protected void Button1_Click(object sender, EventArgs e)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("SELECT * FROM People WHERE Age ");
        switch (DropDownList1.SelectedValue)
        {
            case "=":
                sb.Append("= ");
                break;
            case ">":
                sb.Append("> ");
                break;
            case "<":
                sb.Append("< ");
                break;
        }
        sb.Append("@Age");
        SqlDataSource1.SelectCommand = sb.ToString();
        SqlDataSource1.SelectParameters.Add("Age", TypeCode.Int32, Age.Text);
    }