I had my first when I was 35. A cousin the same age became a grandmother the same year.
Now see imo that is too young to be a grandmother and too young for either your cousin or their child to become parents.
That the equivalent of DN being my child and having their first baby now. When I held DN in my arms at 17, as much as I adored babies and wanted one of my own I knew I wanted to give my children the absolute best. My sibling was mid 30s when eldest DN was born, in a stable marriage, huge house with ready made bedrooms for their growing family and a big garden for them to play in, money for them to have lovely holidays and hobbies and to give them the best start in life with no money worries, no childcare or career worries. They’d travelled in their 20s and 30s, dated around to find the right person and not just the first person who came along, they’d done a lot of work on self improvement. I found it inspirational and knew I wanted to ensure I had everything in place before I started a family. My siblings were born when our mother was 19/20/22 and all felt she was too young and that I (born when she was 38) had a much better childhood then they did.
For me and DN to be a Grandmother and New Mother, both of us would have had to;
a) drop out of education and miss out on professional careers, university etc.
b) be single parents as my “first love” didn’t want to be a 17 year old father and DN has just been dumped at 18 despite them being engaged.
c) live with parents or in a council scheme with other teenage mums
d) live on benefits as neither of our parents could have or would have provided childcare to allow us to work
So yes, that particular scenario is definitely too young to be in the best interests of a child. I wouldn’t have terminated a pregnancy (I made the decision to wait for sex until I was ready to be a parent anyway) but I wouldn’t actively have chosen to become a parent prior to 25 at the very youngest.