Currently western democracies are dominated by 50+ years old, who tend to vote more conservative policies, which benefit them.
Why would we spend very large amounts of the GDP to support old people through health and retirement programs, but refuse to fund quality daycare, schools and child-oriented infrastructure (playgrounds, sports clubs and so on). Especially given that a healthy and educated youth will create tomorrow's prosperity for the country. Unlike the old who have already lived their lives.
If you think that I'm exagerating, I'd suggest listening to what happens in city councils, in retired-dominated cities: they often refuse improvements for families to fund their own programs. It's not a myth, I saw it with my own eyes, and it's very rational.
If a child inherits an estate, or earns significant money, their parents are supposed to manage it until they're adults. Why wouldn't it the same for votes?