Not sure why this is downvoted. The authors themselves note that parenthood could be a catalyst for the change at 30-- in previous centuries, when parenthood happened much earlier, why could it not affect the brain's timeline? This study is simply descriptive of a particular dataset, a collection of snapshots at a particular time and place. Certainly the brain is elastic and responsive to external conditions.

What I noticed is that the 4000 samples are all from England and the U.S. Replicating this study with a greater geographical and socio-cultural diversity would be very useful in supporting or expanding these results.