Yes, in my twenties I had an elderly frail father, living on a pension. I was buying essentials for him on my first salary. I had no parenting myself obviously. He was retired when I was 15, not through choice, but because he lost one job and no one wanted to employ a 65 year old.
Also, and I do think this is probably different now, because there are more older parents and the generational gap is smaller, but I was quite the weirdo in school because my parents brought me up with very few cultural reference points in common with my peers. No pop music in our house. We had Glenn Miller. My parents were born before WW2 and had very different outlooks to my friends’ parents. We did very little exercise because my parents were exhausted. And they could NOT begin to understand a teenager. Teenagers weren’t even a thing when they were one! I was written off as unbearable for things that are described as totally the norm now.
It’s taken a lot of therapy to work through it all a bit. But as I said I think that’s a lot less likely to happen now.
I can relate to this Isitmybathtimeyet - it mirrors my experience almost word for word. My Dad was 45 when I was born and I was his first (Mum already had my sister from a previous relationship). He was a good person and did his best but there was such a huge generational gap. When Mum (4 years younger than him) got very ill when I was 11, Dad really struggled and my sister basically took over.
By the time I was 20, Dad was displaying the early signs of dementia. I'm now in my 30s, he's in his 80s and living in a nursing home. It's hard and none of my friends can relate, as their parents are still reasonably fit and healthy. The main difference I see is that I have been providing support to my parents since I was a teenager, while my friends' parents are still providing support to them now (e.g. emotional support, babysitting, financial help, etc). It does affect you. And yes I know nobody can know how life will turn out and some people die young, but you have to look at the probability. How healthy is your DH likely to be at 70 when parenting a 15 year old?
I don't mean to be unkind to anyone, but this has been my experience and it's tough. I have also had counselling to deal with the fact that I wasn't really able to have a normal childhood and had to worry about my parents from a young age.