snarling and aggressive

Question
My husband and I have had our black lab since she was a pup.  She is now nine years old. She was always a happy, friendly, good natured dog until she turned about three...which happened to be about the time I was pregnant with my first child.  My husband and I did not treat her differently.  She was still a big part of our family.  Over the years she has gotten mean and quite aggressive.  We can't even pet her unless she comes to us first.  She snarls at our kids when they get too close to her.  We have gotten used to saying, "leave the dog alone" just so they won't get bit.  She has her fun with them running and playing in the yard.  She is still very active in swimming and taking walks every night.  I just don't know what to do.  My thoughts are to maybe put her to sleep (I fear there is something wrong with her medically). But my husband is such an animal lover, he won't hear of it.  Is it too late to try and correct her behavioral problems at her age?  Any suggestions?

Answer
I have a lot more experience in starting dogs out right, than fixing old ones.  Dogs can still learn at that age, but it is difficult.  Three is a common time for problems to start.  Much of a dog's personality is determined by genetics and early socialization.  The personality is not fully formed until about 3 years old.  You must have let her feel she was top dog.  As top dog, she feels she has the right to be aggressive and snarl at you and offspring you had no business having.  

Spay her if not already.  It is not a cure all, but can reduce aggressive and dominant behaviors.  Obedience train her. The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog.  Life is much easier if the 2 legged pack members
outrank the 4 legged ones.  You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at http://www.dogsbestfriend.com/

Eye contact is very important.  Meet any aggressive behavior with a firm ''Bad dog!'', and make eye contact.  Maintain it until she looks away.  If that provokes more aggression, try the squirt bottle.  Fill it with water and a little vinegar or lemon juice.  Give it a squirt in the face.  Dogs hate that.