https://www.independent.co.uk/news/science/archaeology/oldest-runestone-norse-language-before-vikings-b2691713.html
Where's Heimdal when you need him....
Ca AD 275 wouldn't quite make it the oldest runic inscription - but then I guess there's no reason the inscription couldn't already be old when the stone was broken and placed in the graves.
Proto-Norse of this era isn't be much like "classical" Old Norse of the High Middle Ages - linguistic change was rapid in the Migration Age (with in many ways parallel changes affecting Scandinavian, West Germanic*, and Insular Celtic - there's probably interesting sociolinguistic conclusions to be drawn here).
* I.e., English, Dutch, German, and allies.