The idea of 16-year-olds like Cavan Sullivan playing for the USMNT was once unfathomable – until Mike Slivinski, Mark Jonas and Nelson Vargas appeared in a 1991 friendly If 16-year-old Cavan Sullivan takes the field for the US men’s national team in the coming days, he will join a list of youngest-ever US players that has no shortage of recognizable names. At the top is onetime phenom (turned cautionary tale) Freddy Adu, still the youngest player to ever appear for the US men. Names in the top…