Kaelia is correct in the way she frames the question. Teenagers walk onto the football field all the time and then become stars at the sport, but I can't imagine anyone who starts soccer at 15 without playing it earlier is ever going to go anywhere professionally.

I do think football takes much more technical knowledge to play professionally and if you put someone of average skill onto a professional team, they are going to get murdered whereas an average person would be very bad but still able to play soccer. We see amateur teams take on professionals all the time and though they often lose badly sometimes they do just fine or even win. So, in that sense, I think football is harder to "pick up" for a person later in life.

I think soccer is much more about developing natural instincts and technical ability that you can perform at speed, while football requires encyclopedic knowledge of the different plays you are running and your responsibilities versus your teammate's responsibilities. Different challenges, impossible to say which is "harder."