I've been conducting an exhaustive survey (talking to a couple of grocery cashiers) about this. And we have arrived at a consensus about the following points.
1. Yes, of course, it seemed as if the season lasted forever when we were kids.
2. Yes, once you get older, summer goes by in a blink.
3. Because their lives are so different in countless ways (communications technology, structured and scheduled activities, near-constant supervision, etc.), children today may never experience that sense of endless summer that once was the norm.
4. The more aware you are of the time of day (as adults and high-tech kids tend to be) — and the more moments during a day that you know the exact time — the faster entire seasons seem to zip by.