Juozas wrote:

> I don't know
> when the Slavic ancestor language split into Western, Eastern and Southern
> Slavic groups but I've read Russian, Byelorrusian and Ukranian
> emerged from
> the Eastern group in the 14th-15th centuries. So the history of Russian
> must've started about 500 years ago. Sergei?

The situation is much more complex and entangled.
First, the very idea of genealogical tree has been deeply compromised - the
proto language usually looks as deeply splitted into diachronically
irreducible dialects, and Proto Slavic is not an exception. Western, Eastern
and Southern
Slavic groups are rather late (X c.?) re-aggregation, the early grouping is
intensively studied, but different scientific schools reconstruct it
differently, if reconstruct at all.
Second, the statement that monolithic and fully set up East Slavic group
splitted into Russian, Belarussian and Ukrainian is rather outdated. A
dialects conglomerate, which is rather conditionally called Old Russian
(VIII -XII c), following one of the theories, can be divided into three main
groups:

1. Krivichian-Radimichian-Vyatichian, showing relationship to 'West Slavic'
Lechitian dialects (both in phonetics and lexis)
2. Polyanian-Drevlyanian-Severyanian-Ilmen Slovenian, showing relationship
to some 'South Slavic' dialects
3. Other (Ulichi, Tivertsy, Volynyane, Duleby, Dregovichi), unclassified due
to the lack of material.

Note that all the contemporary East Slavic languages assimilate
representatives of all the three groups. So if you mean contemporary
standard Russian, it's as young as 200 years (thanks to Pushkin & Co), if
you mean the dialects wich contributed most, it's something about VIII c.

> As to the Baltic ancestor language, it split into Prussian and
> Lithuanian-Latvian groups in the 4th-3rd centuries BC. Lithuanian and
> Latvian began to live their own life in the 5th-7th centuries AD. Why
> Latvian is less archaic than Lithuanian then?

Much earlier, not later than the beginnig of I m. b. c., and consider also
the theory (very probable from my point of view) that to-be West Baltic
(Prussian, Yotvingian etc) group, containing also native speakers of Proto
Slavic, has joined to-be East Baltic (Latvian, Lithuanian and a lot of
exinct Baltic languages in the territory of today's Belarus' and Russian up
to Oka and Volga rivers) at the rather late stage of the Baltic development
(in the middle of I m. b. c.?).

Sergei