For a given wing configuration eg leading/trailing flap, sweep etc, the stall will occur at the same AoA in incompressible flow (low speed, low alt flight) - that's assuming you can clearly define the stall as some fighter aircraft don't have a sudden classical lift break, rather their stall might be defined by rapid drag rise, controllability, handling issues.
For compressible flow, ie flight at high Mach number, the stall AoA will reduce: Stall AoA at 300KCAS/sea-level is greater than the stall AoA at 30,000ft/300KCAS.
Two effects at play here... Compressibility and Viscosity. The effect of compressibility is the predominant one.
Viscosity (Reynolds number). For a given wing design and configuration, as you go up in altitude, the density of the air decreases, while viscosity increases with lower temp. This has a small effect on Reynolds Number.
Compressibility: energy lost in compressing the air is significant at higher mach, which is why you see lower AoA for a given defined stall.
Incidentally, buffet onset is not always a defining attribute of the stall as some jet fighters will experience buffet at very low AoA eg 6-7 degrees, well before the defined stall.