Yes, regret is a big factor, for sure. And like merr7242, I too felt that when I first saw the movie. An awareness of regret set me in motion to change things in my life so that I wouldn't do or create things that I might soon regret. I got a new job (in the same field), moved to another city and changed my vacation plans to do see Alberta this year instead of doing the same old same old.
But for me at least, an awareness of why the movie had such a big impact on me and the connection to the personal losses came much later. I felt the loss of Jack in the movie first of course and knew in some ways it was similar to losing someone I loved, but much later, I realized that there were many more emotional threads that were connected that I hadn't really seen.
In fact, I just became aware of them a year or so after watching the movie the first time. Sometimes the deeper insights come more slowly to some of us, I guess. Oh, and about age and life experiences, you're right on, Penthesilea , right on.
Rayn