Before the first conquest attempts a thousand years ago, the Balts lived protected at the Baltic Sea. Therefore the Baltic languages remained some of the oldest and least changed Indo-European languages. They did have trade connections for thousands of years along the ancient amber roads.
Today the Latvian language is considered younger than East Lithuanian, although that greatly changed from its first recording in the 16th century. The old Prussian language retained the most archaic features. It was written down in the 14th century in the Elbing Prussian Vocabulary.
The Baltic languages have for a long time been verbal languages, the Balts did not use writing until fairly recently.