Ageless is a condition found in items, technology, and buildings left behind by The Forgotten. It simply means that whatever is affected by it can not be damaged in any way. Ageless buildings for example remain untouched by time. They show no signs of erosion. They survived The Pulse without a scratch on them. They look exactly the same as the day they were built.
It is still unknown to us how the Forgotten created this condition for their most important buildings, items, and technologies. Throughout history many wild speculations have been made. From alien technologies to magic. No one has ever been able to prove any of these theories. Perhaps however we were never meant to find the truth.