That’s a common refrain but I don’t see it being relevant. After all, soccer is hardly a juvenile game; it’s also very popular in high school and college.
There is no particular connection between what sport people participate in and what they watch on television. At all. Americans watch football and auto racing in vast numbers, but the number of Americans who actually play organized football isn’t really all that high, and the number of Americans who race cars is relatively miniscule. Far more Americans plays some form of baseball than play football but big time football has made more money than big time baseball for the last 30 or 40 years. One of the most popular Olympic events in the US market is figure skating, but it’s a very niche participation sport.
What’s the most popular participation sport in the United States for adults, not counting pure exercise? Bowling. How many times have you gone to a PBA event? Me neither. I doubt most Americans could name a single professional bowler. But it is both insanely popular AND not identifiably a kid’s sport.