How can you tell the age of a jade plant?

The rate of growth probably depends on the kind of environment it was in. Plants in ideal growing conditions will be a lot larger and fuller than less fortunate plants. The size of the trunk should give it away though. I have a ten year old with a 3/4" trunk on it, but it was in some awful light for a while. If it was in a sunny window for years and years it might be twice that size, oh well.