I found this questionnaire website in which you answer a bunch of questions, and it gives you a fitness age and VO2max estimate.
I tried it out, and its estimate for my VO2max seems pretty close at 56 (Garmin says 57, HRV4Training says 58).
It's part of a study by the Cardiac Exercise Research Group. Any way, here is the link:
https://www.worldfitnesslevel.org/#/