The extension only makes young students more appealing in the labor market in the presence of the laws on mandated health insurance coverage. Without the symbiotic relationship, it literally does not matter.
However, keeping the individual mandate while not extending dependency to 26 requires young people to purchase their own insurance. This would drive down the cost significantly more than having no individual mandate and the 26 rule, or having both. Because it is an individual mandate and not an employer mandate, it does not effect youth employ-ability.
Economics is much more robust than political policy, and the advantages of hiring workers who are already covered under spousal, parental, or senior health insurance should be obvious to everyone. The mandate only puts an exclamation point on existing norms in the labor market.
At this particular moment, it is not customary for companies to offer health insurance benefits to young unskilled workers, but that unfortunate reality is caused by the legislative destruction of the lower-middle class and the healthcare industry. If anything, my argument relies on idealistic market-based labor economics, not the existence of individual/business health insurance mandates.