Split the spending, give them equal amounts, so 1.5 years to 70+ and 5 years to the younger.
The younger will get older, and if they get 10+ years, more of them will lose those years later in life if money is not spent on them when they are older. So spending it all on the young and not the old is a waste of money.
There won't be enough to get old enough to get the 70+ spending if the young are not taken care of, so spending it only on the old would also be a waste because then there will be fewer people to reach that age (and then there will be money available to spend on the young).