As a kid, I ran everywhere - even on errands. As an ambitious teen in uni, I walked very fast like I was stepping on hot coals until I got this girl who wasn't my "girlfriend" but we spent a lot of time together. Then she trained the speed-walking out of me. At first, I was always in front of her without realizing it. Then like a puppy on a leash, she'd tug me back until her speed became my normal walking speed.
10 years later, I still walk at that pace and run out of breath whenever I walk to get out of the hot sun.
As an adult, I'm expected to walk at a measured pace. There are times I'd want to just jump and start running but I can't, cos people would think I've gone mad, unless I have an excuse - rain.
So I'll say the gait speed of adults is more of a measure of how they have been walking for years.