Don't do it like that!
Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
SqlCommand cmd1 = new SqlCommand(String.Format("Insert into employee2 (cemployeeCode,Name,Age) values
(@ID, @NAME, @AGE"), con);
cmd1.Parameters.AddWithValue("@ID", intNextID);
cmd1.Parameters.AddWithValue("@NAME", textBox1.Text);
cmd1.Parameters.AddWithValue("@AGE", textBox2.Text);
That may fix your problem.
But what you are doing there is very, very dangerous.
The only reason for using SQL is to work in a multiuser environment.
What happens if two users try to add a record at the same time? They will both try to insert the same ID!
Either use an Identity spec in your ID column (and let the DB handle assigning a value), or use a Guid instead - there are other ways but those are the easiest.