"Last one to the railroad semaphore at Green Crossing is an old lady!" (54.139)
With that triumphant shout, Jim and Will are off running into the night, and Will's father hesitates only for a moment before racing after them. Despite his 54 years of age, Charles Halloway reaches the semaphore at the same instant as the 13-year-old boys, and "the three of them together left the wilderness behind and walked into the town" (54.49). They have vanquished evil; they are ready to go home.
This ending is multi-layered, just like your last birthday cake. We'll start with all that beautiful icing, which in this case is the triumph of good over evil. All the carnival freaks have scattered, Mr. Dark is dead, and the carousel is broken (hopefully forever). Digging deeper here, what's more interesting is the transformation in Charles Halloway. The middle-aged, depressed janitor is gone. Once a remote and distant father, Charles becomes at the end of the novel an equal and a playmate. He shows us that, sometimes, age really is just a number.
The bottom layer of your cake, the part that you really want to chew over, are the questions this ending leaves for us. The ending shows Jim, Will, and Charles as perfect equals (they all reach the semaphore at the same instant) and as very happy ("exultant"). Yet, just moments ago, Jim and Will were engaged in an enormous fight about the desirability of growing up. Does this ending mean that Jim has gotten over his desire to play Peeping Tom? Similarly, not long ago Charles Halloway was overcome by the old age presented in his own reflection. Does this ending mean Charles has gained a second youth? Like so many happy endings, the end of Something Wicked This Way Comes seems a tad too good to be true.