Question
In S.Central Kansas we transplanted from my cousin's yard in OKC two pecan trees, around 12 years ago. They are about 18 feet tall, healthy looking, but they have not yet produced any pecans.  Is there anything we should do?

Answer
Several thing contribute to pecan production--age of the species (some species of pecan need to be older than others), Fertilizer needs can limit the nut production, Water is needed to have a sound crop of nuts, also pecan production will vary greatly from year to year.
The too things that can be regulated are fertilization and water. here is a web link to information on fertilizing.
http://www.lsuagcenter.com/NR/rdonlyres/FC6A744A-B295-4E41-A47E-750C77D72191/283...

It is best to get a soil test run on your soil this will give the requirements and if the pH or minerals need to be added. Check with the local office of the Agriculture Extension Service for a soil testing kit.
watering the lawn is not enough water for the pecan trees. Deep watering is needed especially if it does not rain. 1 inch of water per week will increase the probability of a larger nut production (place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop).