I got very upset turning 30. It wasn't at all logical, but I'd just had a very traumatic 2nd trimester MMC, necessitating surgery & a hospital stay. I was being very stoic 'these things happen' & think the turning 30 was an outlet for the suppressed grief. (It was my first DC).
By 40 I was separated from my abusive ex for 3 years, 3 small DC, and finding things very tough. I told as few people as possible. I didn't do anything specific - there weren't enough close people to do it with anyway. My family (parents, siblings, ILs) arranged a cake during a family meal out when we were all together on holidays. I was incredibly disappointed. There had been talk of going to this lovely restaurant some distance away, and I was really looking forward to a rare evening away from my DC. In the end my younger siblings with younger DC decided against it & we were in a not-great restaurant with kids everywhere, it was never said it was for my birthday & then this cake arrived, and I realised, this was it! I obviously didn't let it show, but I was so disappointed. If they'd even asked me, I might have said a lunch or something instead.
Now I'm 47, and I'd really like to make a plan now for my 50th (God willing etc) but my family have let me down so much in the last few years I don't think it would include them. I'd probably love to do something with my DC, if I could afford to. Hence needing to plan.
I get the idea of 'big' birthdays being intrinsically important is silly & a bit self-indulgent. But I don't think you can help seeing them as markers for where you are on your life, good or otherwise.