As someone who spends considerable time between 10-18k on Oxygen I have spent some time analysing peoples response to oxygen starvation for various articles (a section in the upcoming PPL/IR book is worth reading).
i have monitored Oxygen saturation levels and arousal in passengers and to be frank at 10,000ft a normal adult will show around the high 80's in Sats without oxygen over an hour or so judgement will start to be slightly impaired and arousal and response levels will fall. At 13,000ft the Sats will be in the mid 70,s and judgement is very impaired.
I always use O2 at 10,000ft and above by day and 8,000ft in the cruise by night.
Also JAA rules require all flights above 10,000ft to use o2 for the crew and have never had the rules quoted above to my knowledge. I think what is being quoted is the requirement to carry supplementary O2 in pressurised aircraft.