That's weird. The guy who sold it to you doesn't sound like he knows what he's talking about. Naturally carbonating a beer is done at room temperature, looks at Sierra Nevada, one of the largest most respectable craft brewers out there, they bottle condition their pale ale, and you don't see all there 6 and 12 packs in the fridge, they sit on the shelves for weeks. Kegging beer and naturally carbonating with sugar isn't something I've done, (I used to bottle condition before I starting force co2) to me that's the point of having a kegging system tho, to force carb n cut out the 2-3 weeks of waiting to drink my beer that I can have ready in about 3 days with pure co2. So no, don't worry about having your beer warm in your keg, your keg is like a bottle, actually even better, because no light at all can get inside, so aging it is fine, keep in mind tho, that depending on the ABV it will age better or worse. Same with hoppy beers, hoppy beers typically taste better the fresher they are. I try to drink my beer when it's nice and fresh unless it's a high ABV beer that I want to age for a while (up to 15 years for some - no joke) but most ales and lagers I do I'm drinking as soon as I taste no more "green" in them. Hope that helps.
- the yeast wrangler