In Europe there is, or maybe was, a custom to call birthdays ending in a zero "round birthdays." They are a cause for celebration. From https://www.howtoliveindenmark.com/podcasts/danish-birthday-traditions/: "But if your friend is turning 30 or 40 or 50, that’s a whole different ballgame. That’s a round birthday. It’s a big deal." It's celebrated with big parties, or a trip.
Five days ago I turned 70. Nothing momentous happened on my birthday -- cards, flowers from family, greetings on Facebook. It was very nice to be remembered. Being very much the introvert I definitely didn't want a big party. It's been on my mind though -- a new decade.
Do I feel different entering a new decade? I have more aches and pains and more medical appointments. But, I am basically very healthy and feel lucky to still be here and able to do so many things that I love and see the people that I love.
It's sort of an artificial border. But people tend to perceive time-related events as significant. So I can think of this new decade as a time for more adventures, time with friends, books to read, concerts to attend, music to play... maybe new adventures?