I can see an 18 being smoothed out across the profile.
Be careful with such assumptions. 12 to 18 years is a big leap in age - more of an exponential leap, rather than a linear progression.
Being "smoothed out" is one thing - certainly, we could expect the more volatile or harsher elements to have been well and truly tamed by 18 years. But 18 years can also introduce excessive oak, and some malts will be dry and tannic at that age, which often isn't too palatable either!
18 years can also smooth out or tame the very elements that gave the same whisky sparkle, life and flavour when it was just 12 years old!