Acting childish seems to come naturally, but acting like an adult, no matter how old we are, just doesn't come easy to us.

The remark plays on the common assumption that “growing up” is automatic. Tomlin suggests the opposite: immaturity is effortless because it follows impulse, while adulthood is a practiced discipline—self-control, responsibility, and emotional regulation—that remains difficult at any age. The line also undercuts the idea that age confers wisdom; maturity is framed as a continual choice rather than a milestone. As a comedic observation, it invites recognition (and self-forgiveness) while still implying an ethical nudge: if adulthood is hard, it is also something we must actively work at, not merely claim by years lived.