It'd be interesting to look at how much of the 20-24 demographic is college students. In the US at least it's pretty normal for parents to pay for their children's phone bills while their in college[1]. Since about half of the 20-24 demographic is likely in college (in the US at least), I'm not too surprised to see that about half of the people in the US have their phone bill payed by their parent.

I'd be interested to see the demographic breakdown as high school, college, or not in school. I think it would offer more insight into a trend.

[1] This is anecdotal. I'm in college currently and most of the people I know are on family plans.