'Established' can mean different things (first settlers, formally founded & named, formally chartered or incorporated, etc.) My town was incorporated in 1870, but founded in 1706, and settled sometime near 1695.
'Neighborhoods' are even trickier, as they're not as formal. You still have issues of multiple dates (when did New York's 'SoHo' start being called that, vs. when was it settled?) but the date and boundaries are much more ambiguous.
Camp Springs, Maryland has been fighting with the County to be recognized, as they're spread across multiple county council districts. Subdivision are easier to deal with, as there'd have been some sort of planning process and establishment of homes (although there may be older homes in the subdivison).
For municipalities, I'm not aware of any one source, but I'd look to either state archives, or state municipal leagues.