No, I don’t think so. If you look back through history children went into the workforce at a much earlier age, sometimes before the age of 10. Attending school until 18 and then entering the workforce full time is a recent phenomenon. I think a more likely factor was the labor force demanding, and getting, wages that were above the bare subsistence level preferred by the capitalist system. Unions and other groups that gave workers a way to exert pressure upon their employers and affect their bottom line through strikes, walkouts and such probably did more toward the improvement of the economic environment than 18 year old employees.
What we have seen since the late ‘70’s has been a concerted effort on the part of owners, stockholders and politicians to quash the unions that exist an keep employees from any efforts to organize themselves to reemphasize profits over people. This is why we see a weaker economy with falling or stagnant wages in these times.