A "last seen a long time ago" could mean a lot of things, depending on the device you use to access Viber and the language set up, but mostly means the user hasn't been online in the Viber application. Also, based on the information provided by Viber, they recommend avoiding status notification because it's influenced by a lot of factors ( an example is the one above). Simply contact that person or call him/her and ask about their presence if you know them well enough.
You can also read what Viber says about the online status and last seen.
