It's unclear at what point adult birthdays become anticlimactic. I think I enjoyed turning 25, but can't remember any of my other birthdays until 30. Thirty wasn't nearly as big a deal as I thought it would be, and it passed without fanfare and nary a glass of bubbles.
Let's be honest here. Adult birthdays are a little bit selfish. I've heard of people who never go to work on their birthdays, insisting they relax and take the day off - as if they're self-proclaiming a public holiday. But everybody has a birthday. It's a universal human experience. None of us are special.
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The crux of the birthday let-down may be that you expect others to make a big deal of yours, but do not exert the same excitement for others' birthdays. Maybe some of us think our friends are closer to us than we are to them? I can understand wanting to celebrate the birthday of a very close friend; somebody you talk to daily and see multiple times a week. Should you really pay the same attention to someone you only see every now and again?
If you are really keen on adult birthdays and don't want them to be anticlimactic, the only way forward is it host your own birthday party at your home, and pay for everything. Drinks, food, decorations ... the lot. And you have to tell people "no gifts". It's still self-celebratory, but at least you're picking up the tab for everyone and facilitating a good time.
Taking any expectations out of your birthday is the best way to deal with disappointment. Then everything (and anything) exceptional that happens on the day is a bonus. We should accept that inside us all is a child who wants to feel special. Maybe we even missed out on such a sentiment in childhood, and think adult birthdays can rectify the situation? Except they can't. That damage, if done, is done.
Birthdays give us opportunities to move past any childhood experiences; especially if you were under-indulged or overindulged once a year as a kid. They're a good day to reflect on the year past and look forward to what's coming ahead, but that's about it.
They shouldn't be used as a barometer of how much people in your life care about you. Your birthday may be a milestone, but it doesn't change how socially successful your life is the day before (or after) it.