The highest level out party ever reached was levels 10-12(which seemed huge).
Killing an adult dragon has only been done once in years of D&D playing (we then brought the local tavern and mounted her head it)
The 40th level example was purely for TOB with its extreme levels but the level restriction was a poor way of balancing out demihumans.
The 3rd ed rules QFLASH are much better as they give good reasons to be any race (including human).