For some companies, the date they were founded is less important than the date they got losted. 
Not that the big companies are bad per se, just they know that as there’s a market for top quality there’s also a market for top quantity that someone is going to fill.
Right now I’m drinking a Feliz Navidad Mole Gingerbread Stout from Buffalo Bayou Brewing, in Houston. I think they’re about 10 years old.
As far as oldest that I really, really like, how about Trappists? Rochefort is the oldest on the list, brewing beer since 1595 or so. But to me, it doesn’t get better as an expression of a style, than Westmalle’s Tripel. I think they’ve been brewing beers since the 1830s, but the Tripel only dates from the mid 1930s or so.
According to the people I talk to:
a) Never fully recovered from the fire
b) The huge numbers of craft and micro-breweries have flooded the market
They look to be on a good rebound but the bad news (for you and many others) is their top seller by far is the mixed packs which they also claim to have invented.
I’ve been drinking a ridiculous amount of this recently. Last year it was only available for one day in our area so every time I see it I figure I should grab it while it lasts. It’s been over a month now.
Warsteiner - 1753.