The factual basis is in the demographics. As the population ages, and seniors leave the workforce, there are fewer people paying taxes, and more people receiving needed benefits such as pensions. How do you think these pensions would be paid without more young people paying taxes? 
Secondly, as people age, they make more demands on the Healthcare system. Again someone has to pay for it. So the options are to greatly increase taxes on young people, increase the debt, cut social programs like pensions and healthcare,,  or increase the tax base. Which option would you prefer?