Why should older workers receive extra protection?
They should not, but doing what you want would probably have a somewhat different result.
Instead of there being a million or so youth in the streets, you would have all of France in the streets.
There would be no law passed then, and the government would fall.
France has high unemployment partly because the law makes it difficult to get rid of employees, thus providing a disincentive to hire
You're right - you would probably have all of France in the streets. But you simply cannot create a second class of citizens and employees that gets treated differently for doing the same work and expect the societal structure to survive. Either France has to do nothing and live with their languishing economy, or they have to remove protections for all and watch the longer-term growth. If they continue to try to create a second class, they're just speeding up another revolution.