The flavor and aroma of the beans continue to develop for the first few days after roasting as the beans release gas (this is the reason some retail coffee bags have valves to let the gases out). The process is similar for bread, with some bakers preferring to let some types of bread rest longer than others.
Right now I am actually drinking a cup of coffee made from beans I roasted this morning, because of poor planning. There's nothing wrong with it but I think it will be better in a couple of days. I find the aromatic qualities peak about 24-48 hours after roasting, but that may be because the off-gassing process is literally emitting the most fumes.
How the flavor changes
Subjectively, I'd say the smell and taste change like this:
During early roasting, there are wood and vegetable smells, like when you put green wood on a fire and it puts out a lot of moisture-filled steam and begins to smoke.
Toward the end of the roast, the smell is more from the browning/caramelization/Maillard reaction and smells kind of like burnt toast. Not really much like coffee.
If you grind and drink it immediately after roasting, it tastes like generic coffee, with those toasty flavors foreground. Very simple and straightforward without a lot of nuance.
After 24 hours the toasted character has diminished and the nuances have developed a lot more. You can really notice the difference between bean origins. There is much more powerful rich "amazing coffee" aroma. I find the sheer quantity of aroma very appealing at 24-48 hours.
Some beans continue to change for several days after roasting. Recently I had a batch of Indonesian beans that were still developing and becoming more characteristically Indonesian after a week. I liked them more, but someone else might have liked them less.
And that, when it comes down to it, is what matters. Some people find some varieties more interesting, more palatable or less harsh as they age. But de gustibus non est disputandum.
It's amazing the human nose and tongue can sense these differences.
See also
- "Degassing, resting, and storage" at thecoffeefaq.com, recommending at least 24 hours
- Sweet Maria's claims coffee is "at its flavor peak at 12-72 hours" and recommends a minimum 12-24 hour rest period.
- forums like home-barista.com report much variation on preferred resting time.