How does aging affect Gouda cheese? I'm particularly interested in how aged Gouda will melt compared to young Gouda, but I'm interested in what happens to its flavor as well. 

I live in the US and have never actually tasted aged Gouda. The Gouda I know is young, soft, very smooth, and melts beautifully. It's almost like American "cheese" that way, but unlike American cheese, it actually has some flavor. I'm interested in a more intensely flavored cheese that is still very meltable.

As a bit of comparison, I use cheddar quite a bit. Young cheddar melts beautifully (in macaroni and cheese, for example) but it doesn't have intense flavor. I much prefer the taste of sharp (or aged) cheddar, but it doesn't melt smoothly. Using aged cheddar alone in mac and cheese doesn't work well; it's grainy and often separates even in a bechamel. I'll still happily put it on a burger though.

I'm about to buy some Beemster Aged Gouda that has been aged for 18-24 months and I want to use it to its best advantage. I'm interested too in 5 year aged Gouda, but with shipping it's very expensive. I'd happily read any comments about the value of long aged Gouda as well.
Aged gouda is my favorite cheese!

Beemster Classic Aged is like a firmer, creamy, sweet, slightly sharp, nutty version of regular gouda.

I've never had a 5 year, but 2 year aged gouda (like Beemster xo) is kind of like a creamy parmigiano but not as hard... nutty, caramel/toffee flavors with random crunchy tyrosine bits.

I would expect the 5 year to be comparable in flavor to Parmigiano with more creaminess and a less crumbly texture due to the fact that gouda is made with whole milk while Parmigiano is made with partially skimmed milk.

Truthfully, it rarely lasts long enough for me to use it in anything... it is the perfect cheese to eat by itself or with apple, pear, or almonds. You can basically use them anywhere you would use a sharp cheddar or parmigiano.