I'm not a great beer-describer, but I'll do my best. From their website, I understand that the best-before date is three years after the brew date, so they are one and four years old respectively.
I don't have two identical glasses, so the younger beer was in a Leffe glass and the older was in my dear Duvel glass. Served at about 5 degrees - 45 for yanks - and allowed to warm up to more civilised temperatures as we went.
Appearance: sparkly, thick smooth head, small bubbles. yellow-gold, nice lacing
aroma: peppery, fruity, yeasty notes. good. as it warms, grassy hops coming out mildly - saaz? becomes sweeter and even spicier; becomes less balanced, but richer
flavour: light, peppery, balanced yeast notes. flavour develops well as it warms.
mouthfeel: light, sparkly effervescent
Appearance: Darker, amber-gold, practically no head, thicker, cloudier. soapy film of head would disappear if not for etching in the duvel glass. no lacing
aroma: heavy, floral, almost cloying. not as nice as the new bottle. as it warms, syrup and floral, candied, gaining a hint of peppery phenolics, nice balance, port-ish now. less cloying than when it was cold. the hops come out more now that its warm, very mellowed, not grassy at all. english?
flavour: sweeter, thicker. not as peppery or sparkly. the fresh-fruitness of the first has become heavier, brandied, syrup. like a nicely aged cream sherry. opens up as it warms. black pepper notes. yeastier. alcohol comes through.
mouthfeel: thicker, heavier, chewy.