Ideally, we can consider that 6 or 7 years old is the right time to step on a tatami for the first time. In a practical way, first of all, because starting at 6 years old, it is possible to progress regularly, passing one coloured belt per year to reach around 14 years old with the brown belt and then having more or less two years to perfect your judo before earning the black belt, one of the objectives of every judoka.
Seven years old is sometimes called the age of reason. This corresponds to the period between early childhood and adolescence. Children become more calm and peaceful (not all, though) on an emotional level. This pivotal period is ideal for starting to understand judo and the values it conveys. Practising our sport will have a great and measurable impact on the behaviour of the children. Is it an exact science? Not exactly, but while the psychomotor heritage of the first years of life is being acquired and before the upheavals of adolescence loom on the horizon, all the recommendations made in the first articles, published recently (https ://www.ijf.org/news/show/what-we-all-need-is-judo and https://www.ijf.org/news/show/why-judo-is-good-for-your -children) are applicable.
It is obviously possible to start judo well before the age of 6. Many countries are setting up 'baby judo' or 'judo discovery' type activities, which allow children from the age of three to put on their first judogi and perform their first rolls and crawls. At this young age, it is often not yet possible to talk about judo in the technical and tactical senses of the sport as we will consider those later. But regarding the principles of judo, the programmes for toddlers already address the big ideas of the sport. Children can learn discipline and respect while having fun and sharpening their motor skills, discovering what the human body is capable of.
If your children haven't had the chance to start young or even very young, that's not a problem either and the advantage of judo is that it can be started at any age in fact. The progression time will not be the same, certain stages will be accelerated, but in the end, it will still be possible to enjoy practising and envision a life in and through judo.
So when you are asked at what age can your children start judo, simply answer, “at any age,” or almost, because whatever the case, they will find a lot of fun in it.