Re net contribution.
If looking at it from purely amount paid in, no age group is a net contributor. Only the richest (who exist in all age groups) are. But they can afford to - and can afford to without losing a still very high standard of living.
Although in a way everyone actually is a net contributor even if not directly through income tax etc. Those many low paid, and also average paid, workers? They're keeping the businesses going and in (often large) profit. And those unable to work? They're also keeping the economy going, through keeping the jobs of the many people in health and social care.
Also contribution to society is impossible to look at on purely financial terms.
And it's a slippery slope, starting to look at who's paid "their share" the most and so who deserves the most back.
I mean, if going down that road then childless and childfree people would justifiably say they pay the most and get the least back. Especially single childless/childfree.
Lower priority for housing and less welfare benefits, no need for schooling (yes, they went to school, but now they also pay for our children to go to school), less use of healthcare (only use for themselves, unlike us parents who use maternity care, and then healthcare for our children).
And no, the majority of children aren't going to become professional carers - so no they won't, when adults, be caring for the childless/childfree. Nor will they be funding pensions - given the increased pension age with the consequence that lots of people will probably die before getting a pension.
To clarify. I am not saying that's a road I want to go down, with that argument of who pays in the most and/or takes the least back. I'm simply giving an example of what happens if we start doing that.