Age can matter significantly for keyword or dictionary word domains - as mentioned above, many older domains have existing backlinks/previous traffic volumes and search engine 'presence' - developing websites based on older domains adds a certain positive 'juice' and can effectively give you a head start in generating traffic and ranking high in the major search engines.
It's why some domains which on the outside can visually appear worthless sell for big money - people are purchasing the 'history' of the name, along with the actual term.
There's also a certain 'prestige' to older domains which increases their value to domain investors - much the same as when you purchase an antique car or watch.
When it comes to brandable domain names, age probably doesn't matter so much - you're effectively selling the domain name on its own merit to a targeted end-user, as opposed to other domainers/experienced Internet marketers. Age can obviously still play a part, but it's much less important than with keyword/dictionary terms.