Marriage is an event in a person’s life.
This is too general because there is no specific controlling idea. The reader has no idea what the paragraph will say about marriage except that it happens.
The average age for peeple in the United States to marry in the year 2000 was 25 for a woman and 27 for a man.
This is much too specific. It gives details that should come later in the paragraph.
The average age for people’in the United States to marry has changed in the past 100 years.
This is a good t0pic sentence because it gives the reader a hint that the paragraph will discuss changes in the ages when people marry. A good topic sentence tells something about the contents of the paragraph but none of the details.