I'm going to answer your questions in reverse.
2. Most craft/small breweries put brewed on dates or bottled on dates on either their bottles, six packs, or case boxes. that will give you an idea of how old they are.
1. Drink IPA's as soon as possible - letting them sit is just letting hop presence fade. I have had IPA's from breweries (DFH, Goose island, New Belgium, Stoudts, New Brooklyn, etc) bottled within a month or less of drinking that claim massive hops but have nothing to them.
If it is a commercial IPA I would say you are looking at weeks if stored cold.
if it's your beer and you dry hop it before bottling/during kegging then the aroma will begin to fade after about a month. nothing will taste fresher than a pint pulled from a freshly carbed(two weekish) and dry hopped keg you made yourself. This sad fact has practically ruined all commercial IPAs for me.
wow. I sound like a miserable putz.