Out of the 4.54 billion years in which the earth has been in existence life took nearly a billion years to come about. The atmosphere at this time was mostly nitrogen and CO2, with water vapour, methane and ammonia declining and oxygen virtually non-existent. After about quarter of a billion years, life began using photosynthesis and from that point onwards life started creating oxygen, however it woud take another 2.5 billion years of photosythesis until there was enough oxygen to allow the creation of an ozone layer without which multicellular life could not evolve into complex enough lifeforms that had what could be properly described as a "nervous system" The reason for this is that without an ozone layer there is no protection against high levels of ultraviolet radiation. In other words sunburn would be so high it would kill off any complex forms of life. Once we have an ozone layer, however, complex life evolves and given another half a billion years and we've got fish. Another quarter of a billion years and we have mammals. So, even though life has been around for quite some time (around 3.6 billion years) as far as intelligence goes, most of it don't mean diddly-squat. Now even if we are willing to accept the possibility of reptilian life developing the equivalent of what one might call culture, i.e. animals grouping together in small herds in which members of those small herds are willing to sacrifice themselves for the other members of the herd, then we could have the level of co-operation between species members to organise themselves into anything complex enough for a civilization to evolve, but the earliest that this could have come about would have been around 250 million years. However, seeing as there is nothing in the fossil record that shows that brain size didn't really begin to develop to large enough ratios for serious intelligence to take hold until the development of simian life, and then only the latter part of those creature's evolution, then I can't see how any kind of civilization could have come about until the arrival of a subset of simians known as the genus "Homo". Even if a civilization arose hundreds of millions of years ago, this wouldn't of occurred overnight, there would have been a gradual development from small brain to large brain and add to that the the level of proliferation that a civilization would create, then I can't see how there wouldn't be sometthing in the fossil record that would indicate any of this. Civilizations may rise and fall as rapidly as over thousands even hundreds of years, but the kind of evolutionary development that this arises from takes millions, if not tens of millions of years and millions of years of evolution creates billions of creatures, there would be something in the fossil record surely. Looking at what the bonobos are capable of, I suppose there's a possibility of simian life evolving enough for the development of civilizations as far back as 2 million years. However, this is an estimation from the standpoint, purely as an educated guess from a laypersons point of view. I may be missing considerable information here relevant to brain, cognitive and psychological evolution. But as you can see, most of the time that life has been around, the vast majority of that time was spent on preparing the earth's atmosphere for oxygen production, 2.6 billion years used up there, and without oxygen, no ozone, no ozone no multicellular life. So I am willing to accept that life could have evolved to a state capable of producing a civilization maybe about 1 million years ago and I am fully aware that civilizations rise and fall, but the only thing that would lead to extinction would be either pollution or nuclear or bio warfare. For a civilization to develop, the species first has to become biologically dominant, otherwise it would be overrun by other species. Any civilization capable of developing that kind of complexity would have dominated the planet much like we have and the level of proliferation of that civilization would have been totally global. We are talking about a civilization that would have taken tens of thousands of years to develop and like I said the proliferation would have been global, so where is the fossil record of a global species that would have grown to at least tens of millions. At the moment the (fossil) record shows only a slow evolution from reptile to mammal to simian to homosapien to homosapien-sapien. Brain sizes don't appear to have arisen until around 250,000 years ago, however, I am also willing to accept that interglacial periods (vast (in that they could disrupt a civilization, but minor compared to a proper ice age) movements of ice between ice ages) of shifting icesheets could have wiped out evidence from a previously arisen civilization, but we are talking within the last quarter of a million years. I'm willing to accept that humans may have developed civilizations over the past few hundred thousand years, however, I have heard that modern humans (homosapien-sapien as opposed to homosapien) had a higher degree of cognitive ability when it came to the ability to conceptualise. For example, a creature may be able to conceptualise to the extent that it can think about itself, but that doesn't mean that it has the ability to think about itself thinking about itself. There is an order of magnitude that differentiates between being able to think and being able to philosophise. Without philosophy there's no mathematics and no engineering or technology, or at least no really complex technology and without that there's no chance of a civilisation taking hold, so, the question of how old can a complex civilisation be, is without doubt a can of nemotode worms that may or may not glow in the dark.