Where's the money?
It's not that young people are not willing to contribute to Open Source community. It's that they can't just do it for free and pay for the rent and food with a ever-growing price tag.
You want more "fresh blood" in your project? Good. Put your money on the table. Offer some full-time jobs for that. No money? Sorry, the young people will get a job elsewhere to make a living. That's how this world works today.