people would treat healthy 20 year olds as more valuable than mature 50 year olds, and the oppposite is mostly true

This isn't true in my experience. The death of a 20-year-old is grieved as untimely, while the death of a 70-year-old is often accepted as the natural order; 50 falls in between (still untimely but not the same level of tragedy as at 20). If it's murder, then you get more sympathy with age for being defenceless; you can see that that is the reason, because it doesn't apply to natural death.

People talk about the value of the elderly for the same reason that they talk about the value of female STEM majors and racially diverse neighbourhoods: to overcome society's ingrained prejudice in the reverse. (It is irrelevant to this phenomenon whether people believe what they say, or even if the prejudice is justified; such comments are still a reaction.)