Age does not scare me as much as getting dementia and not being able to recognize my kids. I think if you take care of yourself you can stay healthy for a good long time. I had my first of two kids when I was 37 so I'm going to be a bit older when they are getting into and getting out of college. I'd like to take a bicycle ride across the United States with them when I'm ready to retire and they are just starting out. That is unless they think bicycling is "stupid".
i'll be 59 later this year and have to say that i **love** the aging process! i've had some unfortunate medical issues that may have been brought on / aggravated by my age, but aside from that ... man, it's great
as for dementia / being able to recognize your kids ... i understand that issue / fear, but i think once you've experienced your wife's menopause - "WHO THE HELL ARE YOU AND WHAT HAVE YOU DONE WITH MY WIFE?!" - you'll be less stressed about it.
back in the early 90's i wanted to cycle the states with our kids, too, so the youngest one (just turned 12) and i rode our tandem down the pacific coast highway here in california - had a great time! i asked our oldest son (then 19) if he wanted to ride with us and he declined ... probably thought it would be stupid. nowadays when we cycle and talk about that trip, he always says, "man, what was i thinking - that would have been a **great** ride!"
as for fear(s), i don't have many, but since my mother, father, sister, uncle and grandfather all died from cancer, i suppose i have a concern about cancer and / or a serious medical issue that could financially destroy our family / survivors.
(edit: oh yeah, chuck norris