Fortunately tax policy can be adjusted to encourage things you want to happen within a society. If taxes are punitive, tax credits for having children would lead to a baby boom based on the same logic.
I'm not sure encouraging more children will help long term though. Eventually we need to figure out how to make society work without population growth.
I don't think it's the case, old folks will outvote the young, same as the property speculators are doing.
i don't think a society that allows old people to vote themselves young people's money just because there's more of them is a good one.