Kegs and getting started

Very few people here cellar sixtels. Mostly because it's cost & space prohibitive but also because it's considered generally inconsiderate to the brewery. Kegs at 50F will age fine as long as they're in good condition physically (good gaskets, tight seal, etc). Bars & breweries age in kegs all the time.

 

like most things, it depends on the beer.

keg beer is typically not pasteurized in America, with Anchor being the main exception. keg beer from Europe that travels to America is almost always pasteurized. it's just too expensive to refrigerate a keg all the way from Europe to a loading dock and then on to your distributor and retailer. this is an important consideration as pasteurized beer is not "better" than bottle beer and many times is arguably worse. pasteurization is a process meant to diminish the ability of micro-organisms to reproduce and spoil food. this includes yeast. so pasteurized beer is dead beer. compared to unpasteurized anyway.

pasteurized beer can be held at warm temps for some time. not hot, but warm. and not forever either. unpasteurized kegs are usually best consumed within 60, 90 or maybe 120 days depending on style and usually must be kept cold or reasonably cold. short spells at room temp won't destroy a beer, but a week at 80 is not going to do your american light lager any favors. do not store unpasteurized beer at room temp, or if you do consume relatively quickly.

cellaring at say 50 will slow down the staling but do not expect the same results as bottle format.

bottle beer is filtered. in the case of bottle conditioned beer, it would be very hard to find a reason to go for pasteurized keg over bottle conditioned. Chimay White is a great example. even still, the vast majority of bottle beer does not benefit from cellaring. which means-

the exception...
alcohol is a good preservative and hops are antiseptic. so in the case of higher abv beers as well as highly hopped beers, the shelf life is greatly extended and this goes for pasteurized and un pasteurized. hops will fade over time but this is true of all formats and part of the fun of cellaring.

conclusion
you can certainly cellar a keg of beer, but choose carefully what you want to experiment with. it is one thing to find your 3 year old bottle of imperial stout isn't so great and quite another to find you have 5.25 gallons of wretched imperial stout.

Cheers.

ps-
your keg is the property of the brewery. there is not time limit on the loan, but you should probably let your distro know if you have long term plans for the keg. this will help to keep you in good graces. they may think you are a keg thief.
Cheers.