I've made 2 gruit ales, and they both kept fine...I'd even venture to say that one of them (heather, mugwort, and yarrow in a Scottish ale base) aged exquisitely! It truly gave credence to the proverb: "Q: When is aged homebrew best? A: Right after you open the last bottle!"
Granted, they were both ~8% ABV, so I'm sure that enters into the equation somewhat...you can age pretty much anything of that ABV or higher and it will be fine...
Come to think of it, perhaps I should do another gruit...I was on a kick there for a few batches during the hop shortage a couple years back, but it's fallen to the wayside now that you don't have to buy 2 batches worth of ingredients to get your LHBS to sell you 4 oz of hops!