18k might be ok for young, healthy folks. Take it down to 10k or so for me. (Yes, I’ve been higher than 10k in a car in Colorado and cable car in France, but I wouldn’t want to try 18k in a plane these days).
Yeah, I have climbed to over 14,500 in the Sierra of California. Difficult to breath properly, I was in my 20s then. It would be different in a plane at my age now 67.