You never want to write code like this because your database can be damaged and/or hijacked by sql injection.
Instead please use code similar to:
sql = "UPDATE Lecture1 SET age = @age, Gender = @Gender..."
cmd.Parameters.AddWithValue("@age", TextBox5.Text);
cmd.Parameters.AddWithValue("@Gender", TextBox4.Text);
...
Also, do not leave the default control ids. Having all your textboxes named TextBoxx makes debugging much harder. It's very quick and easy to give them meaningful IDs: txtAge, txtGender. You'll thank yourself later on.