The boundaries of Decatur Heights are defined slightly differently by different people and groups for different purposes. Regardless of how you define it, all of it (and more) is in the image below.
The original Decatur Heights (built between 1910 and 1930) consisted of Sycamore Drive from Ponce de Leon north to the city line, those streets feeding off it, and those streets feeding off them. The houses north of the city line were added in the 1930-1950 range. The streets off Sycamore to the north of the city line were added around 1950. Winn Way and Aracadia are more recent additions.
Decatur Heights is blessed to have limited entrance and exit points, thus keeping the through traffic down. Planned changes should further reduce through traffic the additional noise and safety risks it brings.