"Older parents" are one of the levels of the pseudo-independent variable Parent Age. In the study the researcher could have classified the oldest 1/2 of the parent sample as the older parents.
"younger parents" is the other level of the pseudo-independent variable. The researcher could have classified the younger 1/2 of the parents as the younger parent group.
This is a pseudo-independent variable since the researcher could not assign the parents to be younger or older. Parents came into the study with their age and all the researcher could do was use that subject variable of age to sort them into their proper group. Don't confuse the pseudo-independent variable survey study tactic of random selection (where the researcher gets a random sample by choosing some small % from a population) with the true experiment's random assignment (where the researcher can decide which group to assign each of the participants to be in.)