Why on Earth would asparagus jizz make them younger? No one remind Bruce that they're not on Earth.

He's face first in one of the water jets just then and only shakes his head, taking his time to give himself a thorough scrub before turning around to continue. "Maybe because I have more experience with the little guy than you do, and I don't think we have the right to tell them that they aren't grown up enough to leave the nursery."

If Earth goes the way of Asgard, Bruce wants humanity to have family out in the stars who'll take them in.