There are kids who start at 5, but they require extra care because of their young age. You should consider several factors before they start: family support (anyone in family plays chess), maturity of the child, availability of a good patient teacher, and peer support. You don't want to move too fast and get them discouraged. The earlier they start, doesn't guarantee, the better they will be. The future will inevitably depend on how much hard work they will put in in next 10 years.
The same discussion applies to older kids. Age in fact is not hard standard in stone. The maturity, the ability to comprehend information and follow instructions, the self discipline, and the personal interest are key factors.
We don't encourage to start at age 4 or younger. Starting at 9 will be a disadvantage, because it will be difficult to log 10,000 hours for study in his/her school years. But all things depend on interest and motivation. We saw some kids started at 9, and became national champion and reached 2300 couple years later.
No matter what age, give them some exposure to chess. If they want to start early, then start early. If not, then wait.