Question
Is the Oak tree the only one that produces acorns. From what I've read Oak trees produce acorns at about 20 years of age. Our tree was planted in 2003 and has produced acorns for the last 2 seasons.

Answer
The acorn, or oak nut, is the nut of the oak tree (genera Quercus, Lithocarpus and Cyclobalanopsis, in the family Fagaceae). The true oaks are the Quercus genera. Other acorn producers are the tanoak (Lithocarpus) and the ring-cupped oaks of eastern and southeastern Asia (Cyclobalanopsis).
Oaks have been bred and cross pollinated to produce acorns at an earlier age for wildlife (saw toothed oak for one). When oaks are fertilized and watered and grown in landscapes they will produce some acorns very early. The data on acorn production is in the forest situation and is dealing with large amounts of acorns not to say some acorns are produced on much younger age trees.
So the age of production will vary with the species of oak and the growing conditions.