Maybe if we compare children (?) from various generations, it's probably true that the overall fitness of today's 12 year-olds is less than that of 12 year-olds of 50 years ago. But, besides the advent of videogames and cell phones, 50 years ago we had physical education classes. Nowadays, phys-ed courses (and music programs) have been cut. Besides, though I don't know about Poland, in the US many of those who were kids 50 years ago are obese now. So, I don't think the physical fitness of today's 12 year-olds will have much effect on their futures.
That said, the present generation of young adults seem more interested in fitness than earlier generations. For ex., there are a lot more of them practicing martial arts. And, let's not forget that they are still breaking world records.
I don't think the difference indicates a decline. Let's face it, how many humans today would survive if they had to climb trees?
Probably the majority of our current crop of 12 year-olds would do better than those who were 12 in 1970. Iow, it's a matter of necessity. Today's youth are no less capable than their predecessors. Though, in fact, today's 65 year-olds are way ahead of previous generations of the same age. Or, at least, they think they are. And there seem to be lots of elderly people who can still outperform people 30 years-younger.
"A man is rich when he has time and freewill. How he chooses to invest both will determine the return on his investment."