Pre-1950s Western countries: Training often began very early — sometimes before 12 months — but this was parent-led training, not child-led. Parents watched for cues and held children over a pot on a schedule. Children weren't initiating independently; parents were managing their elimination on a schedule. This is sometimes called "elimination communication" today. It works, but it requires a full-time caregiver dedicated to the schedule — something that was more feasible in eras before both parents working outside the home.
What "trained by 18 months" actually meant: In many historical accounts, "trained" meant the parent had trained themselves to manage the child's schedule — not that the child was independently aware of and responding to their own bodily signals. True independent toileting — child recognizes urge, walks to bathroom, manages clothing, uses toilet, wipes — developed on roughly the same timeline as today.
Cross-cultural comparison: In cultures today that still practice early elimination communication (parts of Asia, Africa, Latin America), training starts in infancy and children are often out of diapers by 12–18 months. But these systems require near-constant caregiver attention and responsiveness. They're not comparable to asking a toddler in a Western household to self-initiate.
The disposable diaper effect is real, but overstated: Disposable diapers did push training later — the discomfort incentive disappeared. But the idea that diapers are the sole reason for later training ignores changed family structures, pediatric guidance shifts, and a better understanding of child development.
History doesn't support the idea that modern children are uniquely delayed. What's changed is the method, the expectations, and the context — not children's bodies. If your 20-month-old isn't trained, they're on track. If your 2.5-year-old is showing readiness signs and you haven't started, now is a great time to begin.