We weren't lacking teenagers. We were lacking a dirt cheap seasonal exploitable massive workforce. There are not enough teens even if we conscripted them all. And if you don't, teens don't like doing work. Most teens don't live in the sticks anymore. And (for teens) we care about these stupid things like safety, a minimum wage, and labor laws.
It's not 1950 anymore. The world has changed. The reason we need migrant workers is WalMart. Their demand for ever lower prices creates the need for the lowest priced highest supply scalable labor due to a global supply chain and pricing pressure. You can't put the genie back in the bottle. Want cheap shit? Then you hire migrant workers. Or give up on farms and buy from overseas. Or make robots that can pick fruit. There's no free lunch.