The longest I have aged a beer has been 1-1.5 years. I have enjoyed beers from others that have been up to 10 years. With Lambics/Sours/Belgian, the funkiness and complexity really increase. With stouts, it can vary. What often happens is some flavors intensify and some fade. Like if you have smoked pepper barrel-aged stout, maybe after a year the peppers fade but reveal some great roastiness or maple that I did not taste before.