When you're young and lucky you lack both the experience of bad things happening to you and others and, if you're healthy, you think 'well I've always felt pretty much the way I feel now and when I get hurt I heal pretty fast' so you don't think too much about it. Also, most of the people around you tend to be in the same state, though there were probably at least a few exceptions but they were so few that they're easy to overlook.

Then you get older and experience aches and pains where there were none before, when you get hurt instead of shaking it off you either take longer to heal and/or need a trip to the doctor, maybe you deal with a nasty illness/injury or two, see people you know dealing with and/or taken out by nastier things. Throughout all this you start becoming your parents saying things like 'stop running around with that... you could put an eye out' (hell, by this time you probably met someone who did just that)

I figure in a few more years I'll be driving 20mph in the fast lane with my blinkers on... after all, what's the rush? Hey kids, get off my lawn! ;-)