I did wonder about that, but I think probably not. I think somebody decided that 18k is the figure above which if your nose breathing gets a bit lazy then you might "run away down hill" mentally before you realise something is wrong.
At say 16k you just feel knackered and maybe get a headache after some minutes.
But at 20k you have to breathe quite deliberately - I've been there. I have not tried this at 25k (would need a better plane) but it would be pretty tricky, not to mention serious passenger supervision issues.
I've done an hour at 20k and a number of 4-5hr bits at 19k, with one passenger.
Then you have a large spectrum of human fitness, ranging from unable to read the altimeter at 12k (been there to see it) to superhuman types who climb mountains without o2.
I think 18k for cannulas is probably right, 20k if you know what you are doing and you have an adult passenger who does likewise.
The o2 flow rates are also a bit high at high levels and oxygen becomes a real flight planning issue on long trips away - because one cannot bank on getting any refills.