Oaks produce acorns way before 15 to 20 years. When I used to work on windbreak projects for the State of Texas, I saw many red and white oak trees produce acorns by their 5th year.
Hammer, the site you refer to is from Utah and their growing season is about 1/5th that of ours. Much of the literature on the internet is from northern states and sources and do not apply to Texas trees with their long growing season of 10+ months and often 12 months. To a large degree, tree growth and maturity in Texas is a different animal than most all other areas of the U.S.
I did a research project about 20 years ago on the growth of pecan trees in East Texas and here at A&M and we found out that for many years with mild winters pecans trees continued to put on some diameter growth all winter and year long.
When selecting plants, take into account our climate. Just because the label says "needs full sun", doesn't mean Texas full sun in August. Japanese maples are a good example. Hydrangias, Hostas, are others. Many of the "sun-loving" labeled plants, can't take the Texas sun after about 2 pm.
