You are definitely on the precipice of old. If you were diligent about diet and exercise, you can pass among younger folk as not being old (at least until you start talking about apps, social media and pop culture). But that won't last. You will truly be old when you experience all of the following:
1. Can read without glasses
2. Can't sleep for more than 6 hours without having to pee
3. Find that all new pop music is derivative of either Depeche Mode or CCR.
4. Find it easier to get up at 4:30 am than to stay up until 2:00 am.
5. Look up old flames from HS and college on social media and see pics of them with their grandkids
6. Consistently fall asleep sitting up whenever you try to watch a movie at home after dinner.
7. React to any young person's complaint of modern inconveniences with stories about pay phones and collect calls
8. Have a better recollection of historic events from the 1970s and 1980s than the 2000s and 2010s.
9. Can't run for sh#t anymore
10. Ask your waiter at least three questions about the menu before finally ordering the most boring item on the menu.