Question
I have a very old (40+ years) Crape Myrtle that has not bloomed the last three years. It stands about 25 feet tall and has never been pruned or maintained in any way. It otherwise appears to be healthy. (1) Is it too late to begin a pruning program on such an old specimen, and if not, how should I go about it? (2) How far down can it be safely cut back?  

Answer
Joseph,

No it's not too late to prune it.  But, if you want to stimulate new growth, for younger flowering wood, you should wait till the tree is dormant (leaves off) to do the pruning.
So you're looking at late Nov, Dec, Jan.

Next, don't think about the tree as a big hedge that you need to shear.  Instead of heading back (and certainly there should be no "topping"!) you need to THIN it out.

You can remove up to about 25% of the tree branches, without serious harm to the tree.  Remove whole branches, don't leave stubs--they won't heal.   Remove them evenly over the whole tree, don't take 1/4 of one side off.

You're reducing the overall density of the branches, so take your time, and take a step back and squint at it once in awhile, to make sure its even, or to identify areas that are still too thick.

Hope this helps.
Mark in Portland

ps...I know these trees can also get mildew infections, which can effect blossoming.  You might need to spray it in the spring with a fungicide.