I don't think any of millennials will see retirement in a way that it works right now. Not only there is a good chance that automation will make having a job rare, but there is also a good chance life-prolonging treatments will become reality. 

I am fairly sure in 50 years most people will be on 'sabbatical' living off guaranteed government income. That or if you prefer dystopian view - starving in slums without any prospect of finding a job.