I believe that one of the keys to a happy life and successful aging is to have a variety of friends. A good mix of ages, interests, and reasons for friendship provides opportunities for enjoyment of interaction, acts of mutual support and companionship. With our lifestyles these friendships will most likely be both local and long distance. Making the extra effort to build and sustain long distance friendships is worth it.
My 90 year old Mom is a role model for me in many ways in this arena. She has been friends with people of many different ages and so has not found herself in the situation of having no friends left alive. She continues to make new friends of different ages even as her world has shrunk due to decreased mobility. She also kept friendships alive with her high school group long before email and inexpensive phone calls made it easier - they wrote a "round robin" letter for 50 years until there were only a few of the original group left. They supplemented that with in person visits and calls to sustain the connection.
I have not been as good at this as I would like to be. I do greatly enjoy the friendships that are part of my book club of 10+ years and have sustained some very valuable long distance relationships. I've let heavy work schedules and moves across the country interfere with other friendships. This is something I am working on improving as a key to happiness now and in the future.