active male who's now 51; I disagree a bit about it "sneaking up" - at least for me. I noticed a massive decline in just a year or two, where I started commenting "I feel old" a lot more.

The delta isn't so different, but there are lots of things where I feel an inflection from "improving" to "flat or declining" and that is extremely noticeable. To me that's what aging is, and it happens FAST.