Separation anxiety can start at any age. But at least you are aware of it now and can do things to help it getting worse. It does require a lot of work and persistant training though so hope you're up for it.
I suggest you google separation anxiety to help you but here are also a few little tips.
Dogs know exactly when we're leaving the house as we have the same routine every time. For example you turn off the lights, put your shoes on, put your coat on, grab your keys.
Try to do some of these things but not go out. So go and grab your keys and walk around jangling them while you're hoovering. Put on your coat to go and make a cup of tea and sit down and read the paper. You want to do this lots and lots so that the actions in the end don't trigger anything in your dog.
Also when leaving the house and coming home, do NOT fuss. Don't say "Bye Bye, be good" or greet him excitedly when you get home. It's hard not to greet when you get home because that's one of the joys of having a dog. But you really want him to no really care whether you're coming of going.
I'm training one of mine at the moment who suffers and she is also destructive. I found loads of really good protocols on the net. Good luck and remember the hard WILL pay off