I think it depends on if there's things that mean you feel connected to them or their death in a particular way.
The loss of grandparents is something most people have experienced by the time they're in their 30's/40's (though of course that point will be getting later due to the population generally living longer).
I was lucky enough to remember 2 great grandparents although lost them when I was quite young and they seemed ancient to me at the time (though actually they were only in their 70's which seems relatively young now).
My granda on one side died not long after his mother though. A combination I believe of his being in poor health as a result of WWII factors (he was injured and also developed an illness due to the conditions in the hospital which he never really recovered from) and his grief at losing his mother. I know we've no real scientific basis for "dying of a broken heart" but I do honestly believe I've seen it several times, in my personal life and as a nurse.
The first time I lost someone of my own generation wasn't someone I was very close to, a classmate I liked well enough but we didn't mix in the same circles. She died of cancer at 16. Was a huge shock to many, but to be honest I think a big part of many of my friends and my grief was that it MADE us realise we were mortal ourselves - something lots of teens don't really internalise. "Old people" dying was one thing, dying at that age seemed genuinely inconceivable.
We then lost another classmate to suicide a few years later and that brought out not only the sense of mortality again but also anger at the person who died because of the manner of death, even though we knew and understood they were ill and not thinking rationally, it still felt like such an unfair thing to "do to" those left behind especially the person who found them (which they would've known who it would be).
I then lost touch with certain people for a while and then when social media started to become a thing, initially with friends reunited, then later others and over the years I discovered 3 more classmates had died, another to cancer, one in an rta and another suicide. The 2nd suicide hit me hard as I had for a year been quite close to them, I was really shocked as when I'd known them they'd been a really confident upbeat outgoing person (I know this means nothing, just explaining how I felt).
2 more that caught me out (I sound a right bloody jinx don't I?!) were
A chap I worked with, not particularly close to but chatted and got along ok, he was mid 20's (as was I at the time - that's a weird thought he's be in his 40's now) and had 2 young children (crikey and they'll be adults!) and died of an asthma attack. It happened while he was working (he was a delivery driver for the company I worked for) so I was the first one to get a call from a customer, quite irate "where is he, he's an hour late!" Which really wasn't like him, we tried contacting him (pre mobile days so we were calling earlier customers to figure out where he was on his route, which was quite rural), figured out roughly where he would be, no known traffic issues etc ended up calling police as a non emergency - I think at this point we were thinking he was stuck due to someone else's accident or something - think we were about 2-3 hours into him being missing at that point. Then the last customer he'd been to returned home having earlier cheerily waved him off as she was on her way out somewhere, she didn't need to wait for him to complete the delivery just let him in, no indication of a problem at this point. The poor woman found him with the van door open and he was apparently reaching for his inhaler which was on the dashboard. She contacted the authorities and as we'd put in the call about a possible accident hold up etc the police were kind enough to let us know rather than leaving it to his family to have to tell us or have us keep calling them to find out where he was. The main part of that workplace was a factory and I have never heard a factory as quiet as that one was that day and for some time after. It closed completely for the funeral. It was awful. How his poor wife coped I really can't begin to imagine. Very sadly the children were so young they likely won't even remember him (eldest was 3, youngest still a "babe in arms")
My poor mum had me come in the door and just sob so hard I couldn't speak and she didn't even know why! I still don't quite know how I held it together at work!! Eventually I was able to tell her and she gave me the biggest hug ever. (And we're not normally like that either). And this was before I learnt how the poor guy died. That was another shock, I'm also asthmatic and have been since I was little and have had some scares but it really made me start paying attention at my reviews and for a long time I was much better at keeping inhaler close by. (I stupidly forgot once when dd was quite little and I was very sleep deprived and my body taught me that lesson again!). That happened over 20 years ago and still sticks in my mind.
The other was my granda's (the one that died quite young) best friend dying. They'd been friends since their first job together at 14. They had a similar daft, childish sense of humour, but also saw each other through a lot of tough things (granda's illness, my gran had a late mc, he and his wife lost a child in babyhood Iirc to measles, then they had twins and that was hard work etc) and although I didn't grow up near my grandparents geographically we were still close to them and this guy and his wife were always there "in the background" only not as they'd buy us wee treats and play games with us (mum wasn't impressed he taught us not only to play gin rummy but how to cheat at gin rummy 😂), they'd (he and granda) also do things like sneak us biscuits if we were denied pudding as we'd not eaten our dinner - like a mischievous uncle. They'd put on shadow plays for us and do 'who can tell the silliest joke' competitions. Sadly when he died I'd not seen him for a few years as he and his wife had moved quite a distance to live with their grown up children who looked after them (well the wife is still alive so she's still being cared for. Unfortunately she now has Alzheimer's). He wasn't young, it was kinda expected (heart issues) but it still really threw me because I had such lovely memories of him. Also because it was the last real connection to my granda as a man rather than a relative if that makes sense?
Sorry gone on a bit there.
Point is you don't need to have been particularly close to someone to have their loss hit you hard. We're complicated entities and grief can be really weird sometimes.
"Our subconscious tends to react to anniversaries, even when we think we’re ‘over it’." Its weird isn't it? My cousins dog (so I wasn't particularly close to the wee thing) died around an anniversary for me (mc) and I went to pieces! Poor aunt was like "wtf?" The mc wasn't widely known about in the family so she'd not connected it. When I was calm enough I explained - only to set her off because I didn't know said cousin had originally been a twin and aunt had mc one of them early in the pregnancy! So then I felt bad for setting her off!
I don't think there are really any "Rights" over who gets to grieve who, although I do judge people (can't think of the phrase for them) who make their grief very public and all about them, rather than being considerate of those closest to the person who died.