Question
I live in Southern California and wondered when is the best time to trim pine tree?  These trees are very mature, about 30  years old.
Thank you.

Answer
If you are talking about pruning the lower limbs back to the trunk this can be done at any time of the year. The best time would be during the winter months since there would be less sap flow. If you are talking about pruning back the new growth on the ends of the limbs I would not recommend this since it could cause the tree to die back.  

Here is a Web link on how to make the cut when pruning a limb.

http://www.tree-pruning.com/how-to-prune.html