I do think the average potty training age might continue to go back farther, but I dont know how far the limit will go. At some point kids will just not *want* to be in diapers. The thing is, our development does not necessarily need to be linked to our physical age. In fact, as a general rule of thumb, species with a longer childhood tend to have more advanced brains and greater intelligence. We are the most advanced species on the planet in terms of mental capacity, and we also have by far the longest childhood proportional to our full life span. Childhood isnt just the time before you are large enough to be an adult; we could grow much faster. Whats really happening is the assimilation of information at an astonishing rate. Our brains as kids are wired and behave in ways that are very different than when we are adults. Our childhood allows us to develop an intuitive understanding of a complicated world and to be able to solve complicated problems. And our world and its problems continue to grow more complicated as time goes by. It stands to reason then that there is actually a decent reason why our childhood might become extended, either as a natural consequence of our more complicated world, or, eventually (using medical and scientific breakthroughs that are not even on the horizon yet, Im talking centuries to millennia here) by artificially extending the length of our childhood. And if you have a proportionally longer childhood, then presumably most if not all of its stages would be extended equally, so you might have a scenario where people are actually potty training at 18, but they wouldn't be equivalent to our 18 year olds, they would be roughly at the same level of development as our 3 or 4 ish kids. If we are talking a super advanced civilization altering the length of its development, it seems reasonable that they would alter the physical development to match as well.