I believe it's the King Tang/Confucius line. China's good record keeping helped keep the documentation alive.

This is pretty interesting too, although I don't think the family tree is documented in the sense you were going for: http://abcnews.go.com/International/story?id=5386347

From a previous answer I gave to a similar question:

There are plenty of people/dynasties that claim they "descend from antiquity" but there are no 'Western' claims that are accepted by historians and genealogists.

There are a number of 'Eastern' claims that might one-day be accepted, the oldest of which is Kung Tsui-chang who claims to be the 79th-generation male descendant of Confucius (though, probably with some adoption involved).

The Japanese imperial family also has a reasonably strong claim, though their surviving records don't go back as far as their claim does.

The Shias in Islam have a strongly documented family tree which goes back to 600AD. They are the descendents of Ali, the fourth Caliph and are known as Syeds'.

You can have a detailed look in:

1. Cambridge History Of Islam 2) Oxford History of Islam 3) History of the Arabs by P. K. Hitti

Edit:

1. /u/sln26, can I am sure provide more details.

2. Corrected date from 6AD to 600 AD

3. Added sources.

Ireland has some pretty exceptionally long lineages that can be traced through historical sources like annals and genealogies. The modern O'Neill dynasty is probably one of the oldest historically verifiable lineages in Western Europe.

The O'Neill take their name from Nial Noígíallach (of the Nine Hostages), an Irish king who ruled sometime in the late 4th century AD - all of these early dates were written down after being passed on by oral tradition, so lifetimes and dates often fluctuate by decades between different texts. Niall apparently waged a few wars in Munster, Connacht and Leinster, and is most famous for undertaking overseas raids in either Britanny or Britain. Later writers like Geoffrey Keating then linked St Patrick's abduction to Niall's forays abroad, but that's most likely conjecture. During the early medieval period, two branches of the O'Neill dynasty (one reigning in Ulster and the other in Tara) dominated the island until the late 10th century.

Niall was a descendant of another historical/sort of mythologized king called Conn of the Hundred Battles, who apparently reigned sometime around the 2nd century AD. According to the literary tradition, Conn was a powerful king and divided Ireland in half with a Munster king called Mug Nuadat, and gave his name to the province of Connacht. Most of his deeds are related to going to war with Mug, but he's also associated with the mythical figure Fionn mac Cumhall.

Conn's genealogy,and by extension, the O'Neill's, goes back literally millennia. The earliest kings listed are probably fabrications, and if you go as far back as the 1st century AD with kings like Eochu Feidlech (an ancestor of Conn, Niall and the O'Neills) and Conchobar Mac Nessa (King of Ulster in the Ulster Cycle of myths), the line between myth, propaganda and oral history becomes incredibly blurred.

The Katoch Dynasty of India.

It is estimated to have been formed around 4000 B.C. by the ruler of an area around the modern region of Kangra. (In Northern India.) But it is safer to say, that the dynasty had existed at least by 400 B.C.

The dynasty had a member who fought against Alexander during his campaign in India (circa 330 B.C.).

In modern times, during the dissolution of the Princely Order, all members of the dynasty became standard citizens of India, but still have family members who continue the bloodline to this day.

This question may not apply, but were the oldest family lines recorded following the matrilineal or patrilineal?

I mean, aren't there are still a lot of old, noble families in England and other West European countries? I was sure there had to be some detailed recordings of their lineage that went a few centuries back, at least? Would love to read something like that.

Recordings of common people would somehow be even more interesting, but I would think that'd be a lot rarer.

Among Hindus, there is a concept of 'Gotra' which people are associated with till date. A Gotra refers to the male lineage a person descends from. For example, people who belong to the Bhardwaja gotra are the descendants of sage Bhardwaja. This sage is believed to have been born around the time of Mahabharatha (around 2000 BC)

It is a common practice among Hindus to not marry within the gotra since boys and girls within a gotra are considered siblings.

Were out of wedlock children commonly put into the written family tree record?

"Documented" is not probably the best word, but the solomonic/axuma dynasty that begun around 100 ad in Ethiopia, and ended in XXth century with Raas Tafari Makonnen (known aslo as His Majesty Emperor Haile Sellaisle I) does qualify as a long line.