We never noticed our 10-yr-old dog was aggressive until our son was born 6 years ago. She's been barking and growling whenever someone knocks. Is she too old to train?

I'm hoping you can give me some advice on our dog. We own a 10 year old Chow/German Shepherd mix dog. She went through puppy training at Petsmart and we had a personal trainer come to our home for basic commands. We are having problems with aggression. We never noticed any aggression until our son was born 6 years ago. She is very protective of our children and house. If a person rings our doorbell or knocks on the door she goes crazy. She literally lunges at the door trying to get to the person on the other side. We have NEVER had a problem with her biting someone but I'm afraid it's only a matter of time. We recently moved, so we now have a lot of children coming to our house. If a child comes into our house she may bark/growl at them and then go lay down. Other times she'll bark, lay down and then later she will start barking and growling at the child even though he's been in the house for an hour or the fact that she's seem him 50 times in our house. I've recently caught her chasing and growling at a kid in our yard when they were playing. She is very tempermental and unpredictable. She seems to be this way with adults and children. Basically anyone that comes to our home other than immediate family. I have started putting her in her cage when kids come over. My concern is if a child/adult comes into my yard if I'm not outside or one of my children opens the door so she can get at that person. We do have an electric fence but we've never had a problem with either dog getting out of the yard. It has been a very stressful summer. What would be your suggestion? Is she too old to train? Would it be best to get a muzzle for the times she is out in the yard? Thank you for any advice you can give me.

Shelly