Hi "Maineward" (??),
Just as Matt (TreeBay) replied, yes. All bonsai, no matter the age, have to be maintained as bonsai. The good thing is that after they have become established bonsai, their age is far less a factor for them than it would be for the same tree growing in nature.
Ancient trees in nature have the natural characteristics of age, including dead and, rotting areas, insect infestations, a large and extensive network of roots that is dying off here and there, fungal attacks from the root zone, minute traces of nutrients in the environment, etc...
With bonsai, an ancient tree may have dead areas, but the rotting is kept in check so as to bring little or no harm to the tree. We keep them free of insect infestatinons and the roots are often little more than just the healthy, young feeder roots. This offers the tree a very efficient and strong root system without the huge network of old-wood/dead-wood root structures in a network that is spread out over a large area in order to support the tree. And, we can offer an environment rich in nutriment. In short, the elements that in nature continue to "age" the tree and bring it closer to death are either not present or are kept healthy by our machinations.
Just as Matt alluded to, bonsai (that are kept by skilled and conscientious people) may expect a much longer and healthier life than they would in nature. Old bonsai are much "younger" than they appear.
Kind regards,
Andy Rutledge
www.bonsai365.com/
zone 8, Texas