It generally depends on the circles you are travelling in when you're prompted to think about these things.
My mum had me at 25 and was considered an "old mum" by her mother, mother in law and most of her friends who had their first before the age of 21.
I started ttc with ex at 23 and finally had dd at 28 and I was very much considered in the circles I was in at the time to be a "Young mum" as the women I was socialising with were very much career oriented and not planning to even ttc until at least 30's, most were thinking around 35. There were a few exceptions but that definitely was the case for most. I knew I had gynae issues, had already had one mc at 18 (which could have made my mum a youngish gran at age 43) albeit then undx - (that didn't happen until 2nd mc) so I didn't want to risk further possible complications due to ageing.
Cut to we move to where we live now... And I'm an "old mum" around here - massively deprived area, few opportunities, lots of addiction etc I'm easily usually 7-10 years older than dds friends mum's and on the rare occasion I'm not its usually that they have other kids older than dd and were 18-20 if not younger when they had their first. The first friend that dd made when we came here, her mum had her when she'd barely turned 15, what was surprising and I suspect unusual even for here was that she and the father were still together (he was about same age), they've since moved away but they were a lovely couple. They had 2 more after dds friend but big gap between child 1 and child 2.
I'm seen as "old fashioned" and "disciplinarian" by many of dds friends excepting dds best friends mum who was also an "old mum" for here being 26 when she had dds friend and we have very similar approach to parenting.
But even within a group you can get massive variation - among my group of friends that were my "gang" at school (so we're all the same age - 47/48 now) and it's a small group too just 6 of us, I have a "young gran" who had her first at 18 and her dd had her first at 20 so she was a granny at 38, but there's also another friend from same group who had major problems conceiving and staying pregnant who finally got her first child just 5 years ago - so that child is younger than the young granny's grandchild! 2 of the group don't have DC at all, one by choice, the other due to health issues sadly.
Dd is 18 now, has her first serious boyfriend but I've said to her (semi lightheartedly) that while I am very much looking forward to being a granny, I hope she doesn't make me one too soon! I'm hoping I'll at least be in my 50's before I am.
Unfortunately dd has her own health issues which could influence her toward sooner rather than later too - not the same ones as me thankfully but certainly something that could make pregnancy if not conception...tricky. She's concerned that guys her age will be reluctant to have DC under 35 though.
It's not always a choice that's completely within our grasp, accidental pregnancies happen (when I fell pregnant at 18 I was on the pill, taking it fastidiously none of the known impediments were a factor - as a result I didn't completely trust it and so used 2 methods from that point on until ttc)
Infertility and pregnancy loss also happen and not always with warning signs. My friend with the 5 year old never had any problems with periods or other gynae symptoms, her partners were biologically fine according to tests, as was she - so no cause found. But still she wasn't getting pregnant. Eventually got her wee one with a successful Ivf treatment, then not long after having them fell pregnant naturally but mc and it went very badly so now she can't carry another.
12??? That has to be from some kind of abuse surely??
There are pros and cons to being very young or much older parents, under 16 is indeed troubling but then there are serious cons to being a much older mother too - but that's very much defended on mn and I've been flamed for so much as mentioning it is perhaps not ideal - one of which is much older or even no longer living Grandparents.