How to validate a date of birth (age) if minimum is 18 in C#

[Lucfan]

I'm new in C# and my code is as follows and I need help cause I'm getting an error that says:

The name 'date' does not exist in the current context

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;


public partial class ValidationIdentity : System.Web.UI.Page
{
    

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnValidate_Click(object sender, EventArgs e)
    {
       
        DateTime d = DateTime.Parse(date);

        if (d <= DateTime.Now.AddYears(-18))
        {
            Response.Write("Citizen");
        }
        else
        {
            Response.Write("<script>window.alert('Invalid Identity Number')</script>");
        }
    
    }
        
    }

[GaryTexmo]

I'm not really well versed in using C# on a webpage, but the error is pretty straight forward here. You're trying to access an object, date, that doesn't exist in the current method's scope.

If you want to get the user's age, you need to ask for it somehow. Your code that you posted doesn't show that.