So many poor assumptions there. The average life expectancy was a lot less 100 years ago:
http://demog.berkeley.edu/~and... Consequently, people got married earlier because they died sooner; this goes back through the beginning of recorded history, and it was really only in post-WWI 20th century that marrying while a teenager became not just not the norm, but socially frowned upon. Also, look at the drops in life expectancy in 1918 and 1943; what you are seeing it the effects of both world wars and the spanish influenza epidemic in 1918. So life wasn't just short, it was unpredictably precarious in a very real, life-limiting way.
While there are definitely observable fetish aspects to the celebration of youth in our current culture, we no longer marry immediately post-pubescent because, for the very most part, we no longer need to as a practical necessity to be able to have family or an otherwise "full life".
You assumptions on economics are so bad they border on ridiculous. Up until the 1920s, 30 percent or more of the US population were farmers:
http://www.pbs.org/wgbh/amex/t... And yes, as the percentage of workers in agriculture declined, those in manufacturing rose; however, the real economic differentiator remains education, and that trend has only been slowly improving:
http://en.wikipedia.org/wiki/E...