8 Year Old Yellow Lab - suddenly aggressive

Question
Hello:
I am hoping you can guide me in some way as I am puzzled with my yellow labs recent behaviour over the past 3 weeks.
Approximately 3 weeks I had my sprinklers on my front lawn and the 2 little girls (4 and 2 years old)were jumping through them.  Toby was in the house and at the front window and could see them. He started jumping and "snarling" and I could see his teeth ! Once the girls left he was fine.  This was the first time the girls (he has known them since both of them were babies) had ever ran through my sprinkler on my front lawn so I thought it was maybe a territorial thing.
Last evening Toby was in my fenced in back yard and the same 2 little girls came out and started playing about 10 feet from my fence (outside of their fenced in yard but still in their yard) and Toby once again started snarling, showing his teeth and barking (it was an aggressive high pitched angry bark) when he saw the girls.  When the girls are playing in their fenced in backyard he pays no attention to them.  Last night was the 2nd time Toby saw them outside of their backyard and close to "his" territory since they were running through the sprinkler 3 weeks ago.
I am not quite sure what is going on and certianly want to understand what possibly could be going on with Toby.

Thank you in advance,
Tracy Watson

Answer
Since the dog is a senior, the first thing I usually suggest is that you bring him to a vet for a full work up.  Usually a change in personality at that age can be a sign that something is different/wrong.  

You may want to reintroduce the children to the dog because they are growing fast at this age and maybe he doesn't recognize them from a distance.  

Also, if he does get aggressive, try to redirect his attention away from the girls vs. trying to soothe him.  If he knows any commands, it would be a good time to get him to focus on you, SIT, and give a treat for sitting.  

It is possible his eye sight could be failing and the activity he is seeing is disturbing him.