cmd.CommandText = "INSERT INTO person (birthdate) VALUES(@date)";
cmd.Parameters.Add(new SqlParameter("@date", dateTimePicker.Value.Date));
do this instead of
cmd = new SqlCommand(@"INSERT INTO Login(Admin,Password,Age,Gender,Dateofbirth)VALUES('" + textBox1.Text + "','" + textBox2.Text + "','"+textBox3.Text+"','"+ Gender +"','"+dateTimePicker1.Value.ToString("dd/mm/yyyy hh:mm") +"')", con);