Today, most of people die because of some kind of illness that we do not yet have a cure for. However, if that was not the case, how long can human body possibly live for? Is there a theoretical maximum at which our brain will simply refuse to function due to age alone, or can we remain conscious forever if we could simply swap our brains into new bodies without any risk?
Even in perfect environmental conditions we would age. Living is associated with a myriad of deleterious processes, both random and deterministic, which are caused by imperfectness, exhibit cumulative properties, and represent the indirect effects of biological functions at all levels, from simple molecules to systems. Here is a good read on this topic https://www.ncbi.nlm.nih.gov/pubmed/27060562
Even if nothing else was wrong (which is not the case) the accumulation of intracellular and extracellular junk which our systems are unable to dispose of (lipofuscin, amyloid beta, etc) would eventually kill us. https://en.wikipedia.org/wiki/Senile_plaques
As for the maximum age - nobody we know of made it past 125 years yet. https://en.wikipedia.org/wiki/List_of_the_verified_oldest_people