Each of us has a birthday once a year of course. As we get older one year at a time, the luster of a birthday may wear off little by little with each passing year. But the fact still remains that it's the one day a year that is your special day.
I also think that, as we get older and leave our adolescence and early adulthood behind, the daily grind of life quietly and unassumingly takes over -- the demands of a profession and the attention of young children being at the top of the list -- to the point that it becomes tough to keep in touch the way we used to. And this in spite of social media and the ubiquity of cell phones.
That's why I respectfully recommend that you make it a point to wish everyone in your life a happy birthday when it's their special day. Developing that custom ensures that you connect with everyone at least once every year -- and lets them know that they are still important to you.
The alternative is to live an existential lifestyle, forgetting the people of your past, forcing them out of your mind whenever they pop into your thoughts, and self-centeredly waiting passively to hear from them first, unless one of you dies before that happens. That's another way to live life.