Not that it's really on topic for this thread, but the altitude requirement for supp Oxygen is 12,500, after 30min. 14,500 full time. Passengers must be PROVIDED with oxygen at all times above 15,000. I have flown a normally aspirated C172 up to 13,500, and a whimpy ole C152 up to 10,000. Takes awhile, but no problem. Service ceilings for light GA planes is usually around 13,500. You military pilots had(have) a 10,000 foot limit, but that's a different ball game entirely.

As far as airspace limits go...

1200 AGL is ceiling for class G airspace (uncontrolled), unless withing a control zone of an airport having an instrument approach procedure, which is then 700. Victor Airways (low altitude en route corridors) have a floor of 1200 agl to 18,000. Above 18,000 are the Flight levels, ie FL 180 to FL600. Only instrument flight rules aircraft are allowed into Class A airspace (Flight Levels).

There is a simple procedure for alerting pilots and operators of unusually high altitude activities, called NOTAMs, Notices to Airmen.
A simple call to the local Flight Service Station is all that's required to have a NOTAM issued about activities such as high altitude glider operations, UAV ops, tethered balloons, and rockets. Any questions call 1-800-WX-BRIEF, which puts you through to your nearest FSS.

If you regularly fly high altitude with models, you should get yourself a VFR Sectional chart from your local airport, which will display and annotate the airspace in which you intend to operate. Class B airspace terminals extend to up to 30 miles from the airport, Class C areas up to 10. A Chart is helpful in defining the sometimes odd-shaped boundaries of such airspace, in which flying high could be a hazard.