Do you love big bluegill?
It takes roughly 2-4 months to reach that size. Much has to do with the available food.
"Some bluegills begin spawning in their first year. However, most don't spawn until they are two years old. The older they get, the more eggs they produce. A four year old female produces about 20,000 eggs. Bluegills will spawn over most types of bottom and often throughout the summer. Eggs hatch in three to four days.
The tremendous reproductive ability of bluegills can cause problems for the fish pond owner. Bluegills often produce more young than the pond can support. When this happens, bluegill growth is very poor and few fish reach sizes desired by the pond owner. Corrective management is then warranted." - INDIANA DNR
Those fingerlings are from a late-summer spawn last year. Bluegill spawn multiple times throughout the Summer; from April, usually when the first spawn happens though it might happen in March for you, through September a pond could have four or five spawns. Not all of the fish are spawning each spawn, but it's not uncommon to be able to find a few spawning all throughout the Summer.
Small bluegill won't grow much through the winter because there's not much plankton, their primary food source, for them to eat.