I think this comes back to what Finn was mentioning about the lump of labour fallacy. There's really absolutely no evidence that there's a connection between youth labour market outcomes and the size of the labour force that's taken up by older workers. There's not a direct connection between those two things.
So as they retire out, there isn't necessarily going to be a whole lot of new jobs for young people. In fact, they will reduce their consumption levels when they retire, meaning they are not buying as much stuff. That might actually reduce job opportunities for young people. These two things can play off each other so it's not obvious.
I will plug some research being done by my student, Sundip Dhanjal, who is trying to crunch the numbers to tease out if there is a relationship there. She is finding absolutely nothing significant between the two.