Have had a 152 up to over 9,000 in Crete, to get over t ' ills between Heraklion and the south coast (at +33 C on the ground). Would have been a very interesting day if the donk had quit. Even the road below was just a succesion of hairpins with no straight bits.
In Colorado Springs 9,000 is necessary (in a nice injected C172), just for about 1,500 feet terrain clearance over my daughter's house and I was nowhere near high enough to get over the mountains nearby. The Hoosier Pass is 11,500 and the Eisenhower Tunnel between Denver and Vail only a little lower with some of the hills up to 13,000 +.
The circuit at Meadowlake is flown at 7,600 and even at my age I can breath OK.
DGG