Cecil Jacobs is the same age as Scout.
Scout ages throughout the book. We know that Cecil Jacobs is the same age as Scout, approximately, because they are in the same class. Consider the conversation in the classroom when they are in Miss Gates's class when Scout is in third grade.
Little Chuck Little, a hundred years old in his knowledge of cows and their habits, was halfway through an Uncle Natchell story when Miss Gates stopped him: “Charles, that is not a current event. That is an advertisement.”
Cecil Jacobs knew what one was, though. When his turn came, he went to the front of the room and began, “Old Hitler—” (Ch. 26)
There are mentions of Cecil Jacobs before this, of course, but this is the first time they are actually mentioned in the same classroom together. Before that, Cecil and Scout get into a fight on the playground because Atticus is defending Tom Robinson. Cecil lives near Scout and Jem, in town.
Cecil Jacobs, who lived at the far end of our street next door to the post office, walked a total of one mile per school day to avoid the Radley Place and old Mrs. Henry Lafayette Dubose. (Ch. 4)
On the night of Halloween, he jumps out and frightens Scout and Jem. This is why, at first, they think they are being followed by him, and they do not know they are in danger. Of course they do not know they are being followed by Bob Ewell. Lucky for them, Boo Radley has been watching them. Ironically, the kids have a conversation with Cecil about whether or not to be afraid of Boo Radley. Like most kids, Cecil does not know not to fear Boo Radley. Bob Ewell does not succeed in hurting them, because Boo saves Scout and Jem from his kinfe.
One of the colorful characters that populate Maycomb, Cecil Jacobs is a friend of Scout and also somewhat of a nemesis. He adds interest to the storyline, but also moves the plot along, by reminding us what the people in Maycomb think about the trial--even the children are affected.