Hey. I am sorry, but don't worry quite yet! :)
Dogs are all special, and 15 is only the average age that Westies live too! Your dog may live a whole lot longer than that!! Age with dogs is not a one-size-fits-all. There is a golden retriever (which only generally live to 12 years) named Augie that lived to 20 years old! Had to post a picture of this lovely <3
Plus, if he/she is happy and healthy, don't expect them to be going anywhere soon ;)
Even if they don't have much time left, spoil them, I repeat, SPOIL THEM XD Make their last couple of years their best couple of years. Make them happy :)))
Tell your Westie congrats on making it to number 15 for me!! :D