An interesting question. While no two couples share identical perspectives and experiences, we assert it's not just libido--it's making affirmative choices in how to continue life's journey into older age and even elderhood.
1. Gratitude matters. Being grateful for seeing each sunrise, sharing a meal, having less aches than the prior day. Older age, empty nest and so on, gives couples the time to be grateful.
2. Staying healthy together matters. Eating well, alcohol in moderation, health care, and above all doing it for each other matters. Speaking frankly about sexual issues with healthcare providers is important.
3. Accepting changing bodies and accommodating to fatigue, aches, and so on matters. Sex at 2 am is vastly overrated--8 am when rested is frankly a lot more fun.
4. Staying in touch with playmates and going on adventures--resorts, cruises, whatever--to create new connections, that also matters.
5. Making choices to stay exciting to each other--naked cocktail hour, erotic outfits, new toys, etc.--for and about each other matters.
If you sense that there is a focus here on the immediate spouse/partner, you are correct. Other couples sense that enthusiasm and confidence and say 'we want that for us'.