One of the most common questions that people ask about bonsai is "How old is this tree?" There is a sense of awe that we gain from the idea that a tree can live longer than a person, and that properly cared for, a bonsai can do the same.
TLDR: Bonsai can live longer than you! Just keep them healthy and happy and they can be a lifelong companion. Have a bonsai that died? Don't stress - this is an opportunity to learn from your experiences with another bonsai tree.
Trees and Age
The average age of old-growth redwood trees in Northern California prior to early 20th century logging was about 1200 years. That's not the age of the oldest trees, that's the average age of the mature trees that were logged. The metabolism of a tree is slow and steady, and it is opportunistic - while many redwood trees take fifty or more years in an established forest to even reach twenty feet in height, a rate largely due to the shade created by the established canopy; young redwood trees growing in open sun might attain that size in as little as 5 years. For a great book on Redwoods, check out Coast Redwood from Cachuma press.
Trees growing in harsh conditions, like the famous Bristlecone pines of the White mountains of eastern California and the peaks all around Great Basin National Park, will grow so slowly even in open sun conditions that in a human lifetime a seedling may not even reach the size of an adult person. The limiting factor in their growth is a combination of nutrients in the harsh soil, and the limited number of days each year that the temperature is warm enough to allow growth to occur. For the rest of the year, the trees are effectively cryogenically preserved in anticipation of the next opportunity. A great book on this topic is "Timberline" by Stephen Arno, which also contains amazing wood block prints of high mountain trees.
Trees in containers are less capable than those established in their native environments, whether ideal as in the case of the redwood, or constantly challenging like the bristlecone pines. The ideal conditions in a pot should be provided for a healthy and long life as a bonsai. Bonsai are not created by depriving light, nutrients, water or any other required element of growth - bonsai are created and enhanced by the proper techniques of pruning, repotting and bending/moving branching.
Answering the age question can sometimes be tricky, since the question may be asking how long the tree has been a bonsai, or may be asking the chronological age of the tree. Because bonsai are not only created from seedlings and cuttings by careful cultivation and training, but also trained from specimens collected from wild environments, the oldest bonsai may have only been bonsai for a short period of time, while still being a thousand years old in chronological age.
One of the longest-living bonsai with documented history is now in the Japanese Pavilion of the National Bonsai Foundation Collection in Washington DC, donated in 1976 for the bicentennial by Masaru Yamaki. It is more than 375 years old! The story of the tree is well documented; there are wonderful images all over the web and in a book by Stephen Voss; there's even a children's book about it!
Quality of Care
The quality of care and attention that a bonsai receive will play the biggest role in determining the longevity. You may even think of good bonsai care as a lifestyle, similar to the way that many pet owners take advantage of the routines imposed by dog or cat ownership. For a bonsai, daily attention to the condition of the plant, its watering requirements (whether or not you actually add water), sun exposure, wind, and temperature will greatly benefit your tree. Frequent watering, addition of fertilizer and optimization of environmental conditions will keep your plant healthy and growing. When you are not available, ensure that the plant receives the same care, either by another person, or by automation that you have designed.
Is Age important in Bonsai?
Contrary to the common belief that bonsai are all old, it is actually quite common for quality bonsai trees to be relatively young in age. A common goal in bonsai cultivation is to create the illusion of age; using good techniques growers can create this illusion in a as little as 3-5 years. While the appearance of age is valued, it is not the only measure of quality in a bonsai, nor is it necessarily the most important. The quality and desirability of a bonsai tree are determined mostly by the size and appeal of the trunk, and the detail that is put into the branch structure. You may see a tree that is only 10 years old sell for a much higher price than a tree that is 50 years old. The latter tree may have qualities of age that the former lacks, but if the shape is not as visually appealing, then it will not be as highly valued to many bonsai enthusiasts.
The form that a composition takes in bonsai is not limited to the portrayal of aged and battered trees. For species such as Maple, Elm, and Birch, portraying a young tree or a clump of young trees can be equally appealing aesthetically. The key to value in either style of composition is harmonizing the elements to create a composition that looks timeless.
What Bonsai Species Show Age the Fastest?
Japanese Black Pine is prized by many growers because the bark on many specimens develops in as little as 5-7 years. While the branching structure may not even be mature, the trunk of the tree will begin to look aged and interesting. By contrast, Japanese Maples have smooth bark which depending on the variety can show definite signs of age, but they develop more slowly and are more visually subtle than the bark of a pine.
Juniper bonsai develop flaking or peeling bark similar to a pine, but in many cases bonsai enthusiasts remove this bark to highlight the reddish-brown color of the bark underneath. The same practice is used on some Maple, Stewartia, and other species where the color of the bark underneath the weathered outer layer is more highly prized than the flaking or plates of bark themselves.
Special varieties of Japanese Black Pine, like "Nishiki", are propagated specifically for their bark characteristics, and create a "corkbark" effect.
Among Oak species, Corkbark Oaks (Q. suber), the trees that are used to create wine corks, are prized for bonsai because the cork expands rapidly on fast-growing trunks and creates the illusion of size and age rapidly.
Use caution when considering corkbark varieties - the same properties that make them create this bark rapidly can make it more easily removed by rotting, animals, or accidents, leaving the tree with odd-looking inverse taper in some cases. This is of particular note on Pines where the corkbark variety is grafted onto seedling root stock, and the graft union is above the soil line significantly.
The Japanese Black Pine at the start of this article is a good example of a long-lived tree. This tree was imported from Japan in 1915 for the Pan-Exposition and is still alive and well, through four generations of owners caring for it.