When Weeb Ewbank started his National Football League head coaching career with the 1954 Baltimore Colts, he was 47, not the age you ordinarily look for in a rookie coach. But the popular Ewbank stayed in command for the next 20 years, the first nine in Baltimore and the final 11 with the New York Jets. In so doing, he made an impact on pro football that has done much to insure its emergence as America's most popular spectator sport.