I think opportunity to use Maths is an important factor.
I was terrible at mental arithmetic until I worked in a bar. I'm nowhere good as I was as that was 7 years ago.
I can multiply and divide, subtract and add, and use and understand all the obvious extensions of that like percentages.
Big numbers I use a calculator. Repetative operations, I use a calculator; anything complex goes in Excel.
If I didn't have a computer or calculator, I'd be far better at more complex problems. And waste oodles of time doing repetative ones.
If I ever had a reason to differential calculus or quadratic equations, I'd have to re-learn it or use Mathmatica.
Younger people may have never gone through the phase where they became habituated to doing simple maths as they now are surrounded by machines that do it for them. Complex manual maths is now only done by mathematicians and some scientists, and even those people often use machines because it's quicker or simpler, or just easier.
If 'we' with maths educations in or before the seventies had never had to use what we learnt in our lessons, we'd be pretty much the same.