To be fair, a footballer retiring at age 18 would be much like a figure skater retiring at about 13. An 18 year old footballer is pretty much in the very beginning stages of his career in present day football. Messi was already well established as one of the best talents of his generation at age 21 and was widely regarded as one of the greatest of all time at age 25. A footballer's prime years is generally in his late 20s to early 30s. If Messi had been performing at his peak in his teenage years and retired at age 18, people would surely remember him but he wouldn't be the legend he is today. Furthermore, it is absolutely not true that the athletic skills required to succeed in football have stayed the same, but that discussion would get far too long. But this is straying from the point. My point is, and not addressing you, that you CANNOT compare Alina to figure skating and Messi to football.Well, that's the point. That's why you can't compare football stars to figure skaters.
The rules of football and the athletic skills requiered to succeed in it have stayed exactly the same for decades, while in figure skating, the rules are changing constantly and the sport is evolving so quickly (in Russia), that the older skaters will always be catching up to the newer generations, who were brought up: a) knowing that ultra-c jumps are not only possible but necessary and b) training them from a young age.
Another point is that the careers of figure skaters are super short compared to the careers of footballers. Messi is what? in his 30's now? But it took him a 15+ year long career to become who he is now. If Messi had had to retire at 18, I doubt anyone would even know his name.