It's just fear mongering, maybe they are like 10% less productive every decade-group after 60 on average, but it's hard to tell considering people leave the work force at 65. I'd wager that they are more productive on average than 20-30 year olds unless they have dementia.

Having an older population is only a problem if want unskilled labor to remain cheap relative to it's social value. The whole world has seen from America that relying on an immigrant underclass creates too many negative externalities for the price saved, I doubt Japan will copy this plan, they will just bite the bullet, pay more, and then act surprised when NEETs decide to work.