Thats true, but on the other hand, newer operating systems have more code, and more code also means more possibilities for exploiting.
That said, I haven't seen any evidence revealing Mountain Lion to be any less secure than the older operating systems.
That could go either way, you could also say older OS that doesn't get any more testing/QA will have more exploit possibilities as well, like the old unpatched XP systems getting exploited in mere seconds without any user intervention (virtually impossible on OS X/*unix and latest Windows OSes starting with Vista).
The impact could also be mitigated in a way that OS has newer code replacing old code that's more resistant against exploits. Experience and wisdom can lead to better security policies, such as Microsoft making massive security improvements in Windows ever since Vista came out.
It's all about the core functions of the OS, how it was built to be resistant. There is a strong reason that OS X and all *nix based OS doesn't get any viruses that spreads easily.
Nothing in the universe is immune, anything is hackable. The question is how long and how difficult it is to do as much as damage as possible. Both Windows and *nix lately have become very difficult to do either quickly and easily. That's why the exploits have shifted to the web-based instead of OS based, such as Java's applets, Flash, and so on. Instead of massive damage, criminals are more focused on identity thefts and so on.