If a grandparent has ever mentioned, helpfully, that your child's parent was fully trained by eighteen months, you are not imagining the gap. It is real, it is well documented, and it has almost nothing to do with your particular child.
The numbers
Research out of UCL puts the average toilet training age in the 1950s at about 28 months. By the 2000s, that average had climbed to around 37 months. A separate figure often cited from that same era is even starker: in 1957, about 92 percent of children were trained by 18 months. By 1999, only about 4 percent of two year olds were trained. Today, the average sits somewhere in the low thirties in months, with a normal range stretching anywhere from 18 to 60 months.
That is not a small drift. That is a different childhood experience, one generation apart, describing what looks like the same milestone.
What actually changed
Here is the detail that matters most: the biological readiness window itself has not moved. Research consistently finds most children are developmentally capable of toilet training somewhere between 18 and 30 months, the same range it has been for decades. What changed is everything sitting around that window.
Disposable diapers are the biggest single factor researchers point to. Cloth diapers make wetness immediate and uncomfortable, which is itself a kind of feedback that motivates a child to notice and respond. Modern disposables are engineered to do the opposite, keeping a child dry and comfortable specifically so wetness stops being something they notice at all. That comfort is a genuine win in a lot of ways. It also removes a cue that used to speed learning along.
The second shift was philosophical. Through the mid 1900s, training was often approached on a fixed timeline set by the parent. Starting in the 1960s, pediatrician T. Berry Brazelton popularized a child readiness approach instead, wait for the signs, follow the child's pace, do not force it. By 1999 this had become mainstream enough that a public dispute broke out in parenting circles, with critics arguing readiness based training had swung too far and was delaying things unnecessarily. Most pediatric guidance today, including the American Academy of Pediatrics, still comes down on the side of readiness over rushing, because forcing training before a child is ready tends to backfire into resistance and longer timelines, not shorter ones.
A useful comparison: in countries where diapering habits differ, average training ages have stayed much lower. Research on toilet training practices has found average ages around 22 months in Turkiye and 24 months in Jordan, well below the current US average, even though the underlying biology of readiness is the same everywhere. The gap is cultural and material, not developmental.
The finding that should actually change how you feel about this
Here is the part worth sitting with if you are mid process right now. Data on training duration shows that starting later often means finishing faster. Children who begin training under 18 months can take around 12 weeks to complete it. Children who begin between three and three and a half years old often finish in about nine days.
In other words, the extra months you spent waiting were not wasted time. A more physically and cognitively ready child does the same job in a fraction of the time. Slower to start is frequently faster to finish.
There is a smaller, separate research thread worth knowing about too. Some studies have found children who began a structured, potty chair based training approach before 12 months showed lower rates of bladder dysfunction later on. This is a much less mainstream approach and is not a systematic replacement for readiness based guidance. It is mentioned here because the honest answer to what does the research say includes the fact that this question is not fully settled, not because it should change your plan on its own.
What this actually means for you
Your child is not behind. The four year old down the street who trained at two, and the three year old who is still working on it, are both well inside the range research treats as ordinary today. The signs that tend to predict a smoother process are consistent: physical readiness like staying dry for stretches of two hours or more, the coordination to get clothing down, and the ability to communicate the need before it happens, plus a child's own interest, showing curiosity about the bathroom or wanting to imitate a parent or sibling.
Girls tend to hit these markers three to four months earlier than boys on average, which is worth knowing so it does not get mistaken for a problem with a particular child rather than a well documented average difference.
When those readiness signs are actually present, not just the calendar age, that is the moment a structured plan is worth starting, since it is far more likely to go quickly and without a fight. The Complete Potty Training Playbook walks through exactly how to spot real readiness versus wishful timing, and I Can Use the Potty gives your child their own step by step version of the process to read alongside you.