I really don't like the entire concept of the average life span of trees. The reasoning is a bit convoluted, but not really that complicated. Say you start out with a tree species sprouting from seeds. Initially there are thousands of them. Over the first ten years these thin out and the total numbers of tree seedlings decreases dramatically. As time passes the trees tend to continue to decrease in number at an ever decreasing rate. so at what ever age you pick there are trees that are still alive and will continue to grow older, and a much larger majority of trees that did not reach this age. There is no average age at which the trees reach and then start to die off. The trees are dying off from the time they sprout and continue to die off ever more slowly as time passes. They are not like people wit an average life span. If for example 90% of the seedlings die off in the first twenty years, does that mean the average life span of the trees is less than twenty years? I don't think so, but that is the kind of figure you would get if you look at the lifespan of trees in the same way as you do people. You similarly can't say that the average lifespan is some percentage of the maximum known age of the tree, because the last surviving tree may live twice as long, than the second oldest tree. The tail of the population age plot may trail off for a long and irregular length. So there really is no good way to define the average age of a tree species. We can guess an average age for say beech, but mostly that is a false impression based upon the logging history of the area. Because you don't see many beech older than 150 years is related to the fact that the forests were pretty much cut flat in the last 100 to 150 years, so you are seeing a false age distribution.
You could define average age by something like the age at which the percentage of trees older than X is some percentage of the number of trees alive at age Y where Y might be something like 100 years for most species. Again the numbers would need to be determined by aging large numbers of trees at a site that had not been logged to screen out false distributions.
For your tree guide I would suggest listing the maximum know age for trees that have had a reasonable amount of sampling. For American Beech the oldest cross-dates specimen is an 204 years old - this number is very misleading because the tree has only been lightly sampled. An older document by Hough et.. al, reported a ring count age of 366 from the Tionesta River area of Pennsylvania. Lee Frelich has suggested that the tree may reach 400 years in the Sylvania Wilderness of Michigan. Those number seem reasonable to me. There are two places you can look for maximum ages One is Neil Pedersons' Eastern Old List http://people.eku.edu/pedersonn/oldlisteast/
which has accurate ages, but with the caveat that many of the species listed have only a limited sampling and likely the max ages listed do not represent a realistic maximum age for the species, and the ENTS Old List http://www.nativetreesociety.org/dendro/ents_maximum_ages.htm
which as a limited selection of maximum ages that have been reported. The ages here are older and I feel more realistic, but are not cross dated and could contain significant errors.
In any case I don't think the idea of average life span has any merit unless defined similarly to my suggestions above, and I would not include it in your tree pages.
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky