I’d add that my wife is a very good hiker, and routinely goes to 14000’ (like every weekend during the summer), but still sometimes gets altitude sickness if she’s having an off day or is going to a higher elevation than that (it took her a couple of attempts to do Pico di Orizaba (18500’) because she’d get sick partway up and rather not risk it.
The point is, propensity for altitude sickness is not well correlated with general fitness level, and you don’t know until you get there.