"They are both 'fifty-nine and holding' and longtime friends." Would you please let me know how you interpret "holding"? Thank you!

Difficult to say precisely-- either they are only admitting to be 59 years old (because they don't want to turn 60), but are actually older; or they are indeed 59 years old, but are healthy and youthful-looking.

The phrase '...and holding' is taken from NASA countdowns, I believe, where they audibly count the time until the rocket launch ('ten minutes and counting', 'nine minutes and counting', etc.) Sometimes they pause in the countdown for some purpose, and the phrase changes to 'eight minutes and holding', 'eight minutes and holding'-- so time ceases to move forward, in a way.
Thank you Mister Micawber, thank you for your detailed explaination. The content where the sentence appears is about kayaking--they are fifty-nine and holding and both of them like kayaking. So I guess it means they are healthy and youthful-looking. Thank you very muchEmotion: smile