Like the other commenter said, I honestly don't think you know many 70-year olds. I know plenty of people in their 70s that are active, physically fit, extremely sharp mentally, and still working. I'm not saying they have 0 decline, but oftentimes since they have more flexibility and free time, they can use that free time to take better care of themselves and get adequate rest. I'd rather have my kid ride with a healthy, well-rested 70-year-old than a frazzled, sleep-deprived, rushing-to-get-somewhere 45-year-old.
80s, on the other hand, are another story. Everyone I know that was in good shape in their 70s who is now in their 80s showed marked decline, even if they were still fairly healthy and fit. My parents had a fantastic retirement in their 70s - tons of travel, lots of hiking and RV trips to national parks, very active socially. When they hit their early 80s it was tough, because the decline was pretty fast. They're still in good health, but they move considerably slower and just finally "look old" to me.
80s, on the other hand, are another story. Everyone I know that was in good shape in their 70s who is now in their 80s showed marked decline, even if they were still fairly healthy and fit. My parents had a fantastic retirement in their 70s - tons of travel, lots of hiking and RV trips to national parks, very active socially. When they hit their early 80s it was tough, because the decline was pretty fast. They're still in good health, but they move considerably slower and just finally "look old" to me.