Something that came up over on G+ – in the course of discussing how many pre-spaceflight eldrae are still around (answer: lots, even if you discount the deep-submergence infomorphs and Transcendent afterlife, and even if some of them spent a while as frozen heads) – which I repeat here for general edification:
Namely, the oldest eldrae currently extant would be Esitaria Cyprium-ith-Avalae Linlethar was born in -284, which makes them 7,406 at the time of the Core War. (Imperial years – call it 7,322 T-years, once you correct for different orbital parameters.)
For scaling purposes, that’s like having someone born, let’s see, post-Alexander and pre-Rome. Or, since they never had a set of Dark Ages there, approximately 1,000 years before the fully-functional steam engine.
(Non-exo) archaeology and history are a very different disciplines, at least until you get well into the pre-Imperial period.
Time Abyss: The members of the assorted naturally or artificially immortal species of the modern day still haven’t been around long enough for this – although the Empire contains plenty of people who are older than all Earth’s modern nations, and at least a few who are older than human civilization period. With time, they’ll get there.
A few Living Relics dug out of the archives play it straight, but generally lack continuity.
Stronger With Age: Averted. The eldrae, and various other immortal species and immortagen-users, don’t generally get stronger with age, short of grafting in more advanced biomods and other enhancements as technology marches on. (On the other hand, they do generally get more skilled at applying that strength, and remarkably more cunning, devious, and involuted – but like it says, that’s not this trope.)
Intergenerational Friendship: Ubiquitous in Imperial society, given both how very long lifespan is compared to generation (any given generation is a small demographic segment indeed, and therefore… and besides, after the first century or so, does it really matter?).
And also the low birthrate mentioned under Immortal Procreation Clause meaning that if you’re the kid in town, chances are that you’re the kid in town. Children socialize with older generations because that’s the only socialization going on.