I have to disagree with Preben Norholm: Altitudes between FL100 and FL
150 cannot be described as "no problem". In fact, everything above FL
100 has to be considered as dangerous, not for all, but for many persons. Symptoms differ from person to person: fatigue, headache, nausea and so on. You have to try it to find out how your own body reacts. When I personally did my "high altitude flight" during my training I went up to FL
115. My symptoms there were like under the influence of small amounts of alcohol. I felt great, talked a lot and thought I just had invented aviation, in one word: enthusiasm.
Some quotes from my student pilot textbook:
Above 6500/10000 ft: first reactions based on reduced oxygen pressure: slower reactions, fatigue.
Above 13/16000 ft: Physical malfunctions due to oxygen deficit, reduced ability for quick and logical decisions.
So as a summary, above 10000 feet flying alone, without oxygen mask, is considered to be unsafe. And then there may be no more "Happy Landing".