> "x% of what? what is the denominator?"

Another reason the denominator is important is that "parents" are (hopefully) completely a subset of "adults". And not all adults have living parents. So who is counted:

    - All adults
    - Adults with living parents
    - Adults within an age range
    - Adults without children

or what? This really belongs in the title. Without it, as you say, the statistic is meaningless.