These large comparisons are sometimes tough to comprehend. Not surprised that the growing metros lead the list, almost would have to unless they were disproportionately skewed away from that demographic. If anything relative to growth rates they may actually under-index.
I believe metro Philly had a minute negative rate among this group though Center City as one example added nearly 10K among this demograohic in recent years. I find neighborhoods or msaller geography at times more meaningful.
Center City gets younger - Philly.com