I don't think there was really an exact moment, but I can think of a few times that made me more grown up. Buckle in and get ready for a big list :P
First would be when I was about 10 and my grandfather died. I attended his funeral and walked behind his coffin. That to me is probably the first time I had to properly face the harshness of the world. I also kinda represented my grandad I guess by walking behind his coffin and I think to me at the time it was a very adult thing to do. I also had to visit him in hospital a lot as a kid and I think the idea of putting somebody else first is rather adult in some ways and taught me something that is useful in adulthood.
I think each death I've dealt with has "aged" me in some way. Each time there was some new element.
Recently my boyfriend's dog died too and since we were dogsitting with his parents out of the country it was us who had to take him to the vet and have him put down. Since I was the least emotional I kinda had to take lead a bit. It's also one of the big moments for being accepted into his family so that one's a double whamy.
First time walking to school/ getting a train etc by myself I guess would be another one. Then going to uni and living in my own house paying my own bills.
First holiday without family. First doctors appointment without family. Basically all those firsts.
First time calling 999 was recent. I think that was probably a rather big growing up moment cos all the decisions were mine since nobody else was around. Although all I did was call the ambulance, it was a combination of things I was taught as a kid that meant I did that. Actually ended up calling like 3 or 4 times in one month now... kinda hope that trend stops soon.
I think my most recent growing up experience and one of the biggest was surgery and a hospital stay. I had a possible problem where the only diagnosis is through surgery. I had to make that decision myself and arrange things. I have since had another surgery which had complications and landed me with a trip to A&E and an emergency hospital admission. Again, I was having to decide when I needed to go get help, but I also spent some time by myself in hospital which was quite difficult for me. I have anxiety issues and it was a very traumatic experience that landed me in hospital.
As far as what has helped me grow up though I would say a lot of it is my parents, but also scouting. I learned a lot through scouts including first aid and water rescue training and in those trainings I was told "You know more than a lot of adults now so you need to take charge in situations like this". That is probably actually one of the best things i've been told. Until then I don't think i'd realised how much I had grown up. I realised that people older than me weren't automatically smarter or better or more responsible anymore.
It's also been my first experience of socialising with "grown ups" and also acting as an equal to them. I had adult leader training when I was 17 so was a lot younger than everybody else there, but since my units leader was so well known I was involved in a lot of discussions and treated as an equal rather than a kid.
On top of all that, having a longterm relationship has kinda started me off with my own family (even if it is just 2 of us and some rodents) and that is a big deal I think. I'm now the (joint) lead of a unit rather than still being the kid. Feels rather good to have reached that milestone before my older sibling too I must admit. Probably one of the only things I've done before them.