Upstate begins in Westchester. The air temperature changes at around that point. I’ve gone from LI to Valhalla many times and been surprised by the difference in temps.
I live in the Hudson Valley and there are three often-argued lines that determine where upstate and downstate meet.
1.Poughkeepsie - the Eastern landing of the Mid-Hudson Bridge, and the end of the line for Metro-North, roughly 75 miles from midtown Manhattan
2. I-84 - About 15 miles south of Poughkeepsie. I would accept this answer 20 years ago, but Fishkill, Wappingers, the Town of Newburgh and other areas north of the highway have become increasingly suburban.
3. The 845-518 border - the northern communities of Dutchess County, across to Greene County (Ulster County in 100% 845) are all still fairly rural/exurb areas and are considerably different than their downstate counterparts.
Any of these answers works for me. I’ve slowly moved over the years from #1 to #2. A lot of people who live here use #2. As a matter of culture the answer (these days) is more like #3.