I've had a lot of good Scotch Whisky, and I'm still torn on what ones I like best. The 10 years are obviously a little hotter, but to me have the maltiest up front flavor. 12 years have smoother, rounded maltiness, with more complexity.The 15+ year olds I've had were silky, silky smooth, but many less intense to a fault. I've loved, liked, and been ..meh. ..about scotches from all three categories. It seems that the maltiness, complexity, and silky smoothness meet at some weird three way intersection, going opposite directions, depending on the whisky and age of it. I'm blown away at what a master whisky distiller ( or any master distiller) can do, or not do in some cases.
I think your assessments for how scotch ages are pretty close to my experience with a few exceptions. The best scotches I've had are the Balvenie 21 year Portwood and Laphroaig 30 year. The Port barrel aging adds a nice layer that gives some extra depth to a ridiculously smooth Balvenie. And Laphroaig is so intense that it continues to develop more complexity well beyond the 15-18 year landmark without losing too much of its big peaty, smoky quality.