Children are young human beings. While there is no specific age cut-off to being considered a child, typically at some point in mid to late adolescence a child comes to be known as a young man or young woman. Many cultures and traditions do have rites of passage that occur at a defined age, after which the person is considered to have full adult status, but this is more symbolic than practical in today's modern world.
Adoring parents will typically refer to their offspring as "their children" even when they are quite old, and perhaps have children of their own.
Grandchildren are one's children's children.