I think a useful exercise for considering the risks is to look at your own parents and their siblings. How is their health as they approach old age?
In my case, I have two parents and seven aunts and uncles:
Parent 1 - started showing signs of dementia in 60s, had stroke at 72 and dementia progressed more rapidly after that. Now 80 and in a nursing home, hasn't known who I am for years.
Parent 2 - 75 years old and her main problem is mental health, though that has been present most of her life. Physically, she has arthritis and finds it difficult to walk for more than a few minutes. She also has high cholesterol and high blood pressure, which she takes medication for.
Uncle 1 - Got cancer at 58 and beat it, but was left with ongoing health complications. Got ill at 69 and died from pneumonia.
Uncle 2 - Major heart problems. Needed a quadruple bypass at 68 and lost a limb shortly after.
Uncle 3 - 72 years old and doing well. Some issues with high blood pressure and cholesterol, but nothing out of the ordinary.
Aunt 1 - developed dementia in her early 70s.
Aunt 2 - 76 years old and doing well, no major health issues. Still active and enjoys travelling, but has had to slow down recently.
Aunt 3 - died at 45 from cancer.
Aunt 4 - 70 years old and reasonably healthy.
So in my example:
- 3 out of 9 remained healthy and active into their early 70s.
- 2 out of 9 died before 70.
- 4 out of 9 have significant health issues for which they require assistance.
I don't think my family is that unusual. If you widen it out to include my aunts and uncles' spouses, you get a similar picture.
If OP's partners family had a similar profile to mine, he has about a 1 in 3 chance of being healthy when the child reaches 18. Of course, most parents want to stay healthy beyond that, to see their children establish their own lives and to meet their grandchildren.