Most scholars believe that Ukrainian emerged around the middle of the 1st millennium CE, after the breakup of the Proto-Slavic ethno-linguistic community, when a distinct Ukrainian ethnic group began to form on Ukrainian lands. It is important to remember, however, that historians of the language distinguish between the written language and the living spoken language, which—though closely related—arose at different times.

The living Ukrainian language developed from ancient Slavic dialects spoken by the population of what is now Ukraine. In the 4th–7th centuries these tribes belonged to the Antes union and communicated with one another—precisely there the Ukrainian language began to take shape. Scholars have shown that at that time there were no Slavs at all on the territories of modern-day Russia or Belarus. Modern Ukrainians can understand Poles, Bulgarians, and Slovaks largely because Ukrainian has preserved closer ties to other living Slavic languages than, for example, Russian has.

The written Ukrainian language appeared in the 10th century CE under the direct influence of Old Church Slavonic (adopted from the Bulgarians along with Christianity). This early written language is called Old Ruthenian because in antiquity the name “Rusʹ” referred to what is now central Ukraine—specifically the Kyiv, Zhytomyr, and Chernihiv regions, as well as parts of the Cherkasy and Poltava regions. (Later, from the late 12th–early 13th century, the name “Rusʹ” was also applied to western Ukraine, particularly Galicia.) The Old Ruthenian literary tradition absorbed features of everyday speech and thus laid the foundation for Old Ukrainian written language (13th–18th centuries).

By the 14th–15th centuries Ukrainian already possessed all the traits of a full linguistic system: stable phonetic, grammatical, and lexical features; written monuments with clear Ukrainian characteristics; and a complete separation from Old Ruthenian. The end of the 12th century is conventionally seen as the point when the phonetic-phonological and other core features of Ukrainian had fully formed as an independent linguistic system. Most of these features are abundantly documented in the earliest written records of the 11th–13th centuries (notably the 1073 chronicle and the graffiti on the walls of Saint Sophia Cathedral in Kyiv) and appear systematically in later sources of the 14th–15th centuries and beyond.

If you are interested in Modern Ukrainian, linguists date it from the late 18th century to the present, although features of the new (modern) language began to appear in the second half of the 17th century.

So, if we speak of the beginnings of its formation, Ukrainian is over 1,400 years old; if we refer to its completed shape as a separate written language, it is over 800 years old. Both dates are scientifically grounded and reflect different stages in the development of the vibrant, distinctive Ukrainian language.