Just wondering how a bottle conditioned beer will change over its life - if at all? And is there a perfect time to drink one? Also do different types change differently?
Quite the opposite, bottle conditioned beer is perfect for aging. The yeast carbonates the beer and will produce slight changes over time, like upping the ABV. This needs to be done in climate controlled environments similar to wine. The yeast will settle to bottom and you want to avoid pouring this into your glass when finished.
I brewed a Imperial honey stout and aged a few bottles at 68f for about a year for my birthday. The beer started at about 7.2 ABV The beer finished at over 9 ABV (this is because the yeast had plenty of starch to munch on and the carbonation was not affected.
In addition to Waitkus his answer I would like to note that the perfect time to drink a beer depends on the type, brand and most importantly taste. There is also a difference when storing larger bottles. For instance a 75 cl Duvel bottle (or larger) can be stored easily for several years, whereas the smaller bottles of the same beer don't fair too well.
Other beers, mostly heavier abbey beers allow for prolonged storing. For instance an Orval changes taste quite drastically once you have stored it for about 1 - 2 years. I've even seen these beers being stored for 30 years and still being drinkable (although you needed to pour it very carefully as there was a lot of cediment). The monks of Westmalle even have 35 year old beer on site for their own consumption.
In regards to taste, they tend to become heavier, stronger in taste, sometimes a bit more sour. They also loose a bit of their carbonation (very old beers).
Bottle conditioned beers definitely have longer shelf-lives than their pasteurized/filtered counterparts - the yeast helps scavenge oxygen and release sulphites to stop free radicals (such as from lipid breakdown) from staling the beer.
The yeast can survive for decades - Guinness observed yeast still alive in the bottle after 35 years!
How long a bottle conditioned beer can be aged for and, thus how soon you should drink, comes mainly down to
- the alcohol percentage - stronger beers age better, and
- the color - darker beers age better than lighter ones
While a light beer may be best consumed within 5 years, darker beers can be aged much longer than that.