Dill Harris has been through a lot for a kid "going on seven." He's been sent away from home to live with his Aunt Rachel, and doesn't have a father. When Scout brings this up, he simply lowers his head, but Jem quickly corrects her rudeness. Unaware that some children have fathers who have left the family, Scout cannot comprehend the level of discomfort in the situation, although Jem has a better idea.
Later on, Dill sneaks on a train to come visit Scout and Jem, traveling to Maycomb, Alabama from Meridian, Mississippi, all by himself. This shows that he has some maturity. Not to mention the fact that he's seen so many movies at the theatre using the money he won from the "beautiful child contest" in Meridian.
The "I'm little but I'm old" line says a lot about his life experiences, even though he's under the age of seven at the time we are introduced to him early in the novel.
When Jem and Scout first meet Dill, he tells them "I'm little but I'm old" (Lee, 6). Dill's comment applies to his plethora of experiences and portrays his complex personality. Even though Dill is only six years old, he has been through a lot. Dill is from Meridian, Mississippi and spends his summers in Maycomb with his aunt. Dill does not have a father and has been to the movies many times, which is something that impresses both Jem and Scout. Dill offers a unique perspective on life since he has been raised without a father. Dill has experienced loneliness at a young age, which is one reason why he continually tells elaborate lies to gain the attention and admiration of other children. Dill is also an experienced traveler who commutes across state lines every summer. The fact that Dill has seen several popular movies also applies to his earlier quote about being old. Dill's difficult childhood, transient nature, and variety of experiences apply to his initial comment.