Simple....the extra 15 feet per second is often the difference in getting stopped or not. Assume that your reaction time to a blow out or strong wind gust is 3 - 4 seconds to see, react and hit the brakes. You've just eaten up 45 - 60 feet. Not sure younger driver vs older driver makes a lot of difference. Younger seems to be invincible, older may have slower reaction time, but it probably all equals out. Younger minds wander the same as older....have been both and can confirm this.
This discussion is one I've never completely understood. Folks get all worked up over inches or a mile or two per hour, when what matters is your safety, your family's safety and the others on the road. IIRC, there's been plenty of research done confirming that lower speed does save lives. If you need to be somewhere at exactly 4:00 and need to drive 75 to make it, you should be leaving earlier. The past couple of years DH has asked me to do virtually all the driving. He's amazed at all the things he never saw before. Not because he didn't have the opportunity, but because he had a very heavy foot - NOT towing. I generally tow right around 62 - sometimes with cruise, sometimes not. I'm far from being an aggressive driver, but not a poke-along either. The only ticket I've ever had was for speed - go figure....57 in a town that was only 3 blocks long and I swear there were no signs. I watch down the road, look for possible problems like a semi flying by and try to be prepared for it. It's very relaxing for us. I feel much more in control. Take it easy, enjoy the drive, you don't know what's around the next corner.