Gentlemen,
I'm a freelance rocket scientist.
An airliners maximum cruising altitude is determined by the lowest of the following three characteristics.
1) The max certified altitude which is usually determined by the pressure load limits on the fuselage.
2) The thrust limited altitude. This being the altitude at which sufficient thrust is available to provide a specific rate of climb...normally 100fpm
3) The manoeuvre limited altitude. This is the altitude at which a specific manoeuvre margin exists prior to the onset of buffet. This varied depending on the certification authority (CAA/FAA/JAA) but is normally between 1.2g and 1.4g giving between 33 and 44 degrees bank angle prior to buffet/stick shake.
Aircraft which can fly higher than the normal 40,000 foot approx ceiling are usually either very fast, (M0.85+) or capable of flying very slowly (U2)
I shall be doing a dissertation on particle physics and the origins of the universe next.