I'd like to point out that at least the Kyuubi holds a decent concept of how old he is.
http://www.mangareader.net/naruto/567/9
He calls "B" a "brat". The original word used is similar to "naughty/annoying kid".
What's this stuff about power of the Bijuu not being tail dependent?
That's not true.
It is tail dependent:
http://www.mangareader.net/naruto/567/10
But this confirms another theory I already had: the biju's strength is based on their tails AND the Kyubi is MORE than twice as strong as any other Biju. Why? Because the Kyuubi is at half power yet he is still stronger than the strongest of the Biju and it IS tail dependent.
Assume everything is in a perfect ratio. Each number represents their "power level".
Shukaku - 1
Nibi - 2
Sanbi - 3
Yonbi - 4
Gobi - 5
Rokubi - 6
Nanabi - 7
Hachibi - 8
Kyubi - 9
But that's how it is, now. But let's add them up when the Kyubi is at full power.
He is an 18. The rest added up equal 36. The rest added up are roughly twice as strong as the Kyubi. This does not explain why Hashirama would sell off the biju. The Kyubi was stated to throw off the balance of the Gedo statue so it had to be sealed last. That would indicate that it is equal to all the others with just it's yang chakra (meaning, his yang chakra equals the total yin chakra of all the others making it "balanced".) So the full power kuubi should be equal to a "36" in tailed beast power levels. The half powered should be an 18.
This makes perfect sense in the context of the manga. The Gedo Statue, why Hashirama thought selling away all the tailed beasts but keeping the kyubu was fine (because it equals all the others put together at full pwoer) and why the 10-tails most resembles the Kyubi: half of the 10-tails chakra comes directly from the full powred Kyubi.