Could be a touch of deafness - they cannot hear themselves barking and tend to not hear you when you tell them to shut up! You might need to use a hand signal to indicate that you are not impressed - I have an oldie of 15 too who does this. If she is barking I throw a tissue or something to land in front of her, just to distract her from barking.
I agree with lankeela ,Iv two oaps and they are the same.We do the distration bit and it seems to work not only hearing but eyesight can play a part in it too.
My old boy does the same thing maddie and he has never been a barker as such ..only when someone comes to the door .I agree with lankeela and garner .Shaney sometimes starts barking for no reason and I just distract him with a toy or biscuit . I think that he gets confused as his sight is going and probably his hearing (bit like me )and it's just old age .I put it down to what I call Dogzheimers !
His latest trick is to get me up in the night just to sit and stare vacantly round the garden for about half an hour while muggins wanders around with him in her dressing gown freezing her tootsies off ! The things you do for dogs !
Yes, my old Greyhound was the same. Would start barking for no reason at all, even in the middle of the night. Got really irritating at times, but almost impossible to stop. 8-)