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Viconia DeVir.

Viconia is possibly the eldest companion in BG1 + 2. Well, Hexxat I suppose, but I consider Beamdog additions less canon, and I have no idea how old Korgan is. Usually we're not given much information on which to guess their age. Like with Aerie we just know she was captured by slavers when she was still 'very young', and a vague suggestion she might be similar in age to Jaheira (which for an elf is indeed very young).

In the Drizzt novels DeVir is the house that's destroyed the day he's born, meaning Viconia was a priestess of Lloth and fled Menzoberranzan before that happened, so she's probably at least a century older than he is. I know that doesn't seem to mesh with her portrayal in BG1 as a totally naive fish out of water. Honestly, I don't think they really thought that much about it; the BG1 companions are more catchphrases than actual characters so they probably just figured DeVir was a neat little reference to throw in.