Teddy did the same thing, at about the same age. The vet found no reason for his behaviour, so it's been an uphill slog to sort him out. He started objecting to grooming and being picked up, also stealing things like socks and defying anyone to take them from him. I decided to ignore him, as the growls seemed like bravado grumbles from someone who wanted to be boss (Izzy was boss and never challenged), but one day he bit. He would attack, but not bite Joly, who would shrink away (Teddy seemed to enjoy this) or layed back Monty, who ignored him. If he jumped onthe bed or a chair, he would growl defiance at anyone trying to move him.
We started strict discipline. He was put straight into his crate in the bedroom, so he didn't get chance to jump on the bed. If he got grumpy , he was put into his crate ( I know people say that a crate shouldn't be used as punishment, but he had to go out of the other dogs' presence). After a few incarcerations, though, he would take himself off ( closing the door), to cool down. He only needs to be told 'Crate if you're grumpy' and off he goes, coming back when calmed down.
I started putting him into sits and downs, with a small treat, sometimes, so he was obeying me and not vice versa. The one thing I still will not do is take something from him. He is offered a treat or made to think I've dropped a treat , as distraction and then I grab. I also wide-eyed stared him out, if growling, sometimes baring my teeth, too and made sure he was first to look away.
Thses things won't work for everyone, but they worked for Teddy. 95% of the time he's fine now. He has the odd grumble and is told to go away if he growls for more than a couple of seconds. If he objects to being picked up or examined, he is talked down and then I proceed, handling so he can't bite, but he hasn't actually tried since the one incident, nearly two years ago. I've learned to predict a mood, by his body language and so don't get into confrontational situations. Little problems are that he will growl at a dog moving in the dark (Is he being aggressive or not sure ?) and that he 'talks' in growls, so we (humans) sometimes tense, then realise he's only talking.
I now have to work on his attitude outside, as he can be charming, but some days, will grumble and lurch at just some people and dogs (black ones usually).
He is actually calmer since Izzy's passing, as he is now top dog, in the same way as Izzy ie in a passive way. This has shown itself in the way his tail is carried , like Izzy's was, up and curled, when he's moving about with the others. He won't be challenged , because Joly and Monty aren't interested in leadership.