FAQ - Answer
Unfortunately, there isn't a precise, clear answer. Tree density is a function of tree species, soil types, ground cover type, percent slope, aspect (north or south), and past land use practices (tree harvesting, grazing). The number you have (120 tpa) might well be an acceptable figure for some areas but I would not consider it a number that can applied across an entire county given the variables I mentioned.
In most cases, people would use available technologies (aerial imagery, Google Earth, etc.) or ground truthing to determine tree density for a particular area as you indicated for the ¼ acre. However, take caution when using that number for other sites unless more information was included. A source of county-based oak information is the Calfire Forest and Range Assessment Program (FRAP), at http://frap.cdf.ca.gov/ . I would suggest you search this site to see if you can find the type of information that may be useful for you.
As for reproductive maturity for oaks, again a number of factors come into play (i.e. species, soil, and age). Oaks have the ability to grow over such a large spectrum of soil types that there isn't one set number that can be applied for either determining age or reproductive maturity. However, it has been estimated that oak maturity is about 30 years.