[OgsyedIE]

Sounds entirely accurate. Among all other technological advances, the biggest tracker of the very first wave of women's fertility decline in rural Europe in the late 1800s was access to electric lighting, which permitted rural women to make use of night hours for self-directed social, political and educational activity. (The source for this I use, although there are others, is The Subject of Virtue by Laidlaw)

Smartphones massively reduce the barriers to entry for self-directed career-based, social, political and educational activity (plus entertainment, but gambling addicts have differing fertility patterns so lesser degrees of it are useless to study). Outside observers may consider the real-world quality of such activities to be low, but the activity they enable people to do in their many brief lulls of free time between different daily tasks do fit into those buckets more than anything else. And the cumulative effect of all of them is to delay life milestones.