The youngest age to go to jail is 8 years old.
However there's really no age limit as to when a child an go to jail for a crime they've committed.
The youngest though to be charged as an adult is 8 years old and be sent to adult jail but they would usually be kept in isolation away from other inmates.
Most kids though younger than 15 would be sent to Juvenile Hall or a youth shelter type jail until they were old enough to go to regular adult jail or be released.
Kids as young as eight can still be charged as an adult, held in an adult jail, and sentenced to extreme sentences in an adult prison.
Even a 15 year old or 14 year old who commits a crime can be sent to regular jail and charged as an adult.
Although most states allow a juvenile of 8 years old to be sent to jail, it is only in rare cases that they are sent there.
However, in some states, there isn't an age limit for a child to be sent to jail.
In fact, the decision is left up to the judge to decide.
Using handcuffs on a child or any other kind of physical force should only happen when absolutely necessary, like to stop that child harming themselves or someone else.
It should also only happen as a last resort and for the shortest possible time.
Most Kids younger than 13 or so don't get handcuffed unless necessary as I've seen some kids get arrested but not handcuffed.