Age Guidelines for Youth Runners
Children naturally run soon after they are able to walk. However, it is important to advise children to not run too far or often at too young of an age. Most half marathons and marathons have minimum age restrictions for competing.
Here is an excellent guide by Road Runners Club of America (RRCA) on running guidelines for children.
Age Guidelines for Older / Masters Runners
It is never too late in life to begin running. It is important once one ages beyond the prime running years, to take precautions to not incur medical issues related to overexertion in running.
Here is an article that provides helpful tips for those beginning runners that are beyond 50 years old.
This article provides tips regarding running as you move through the older age groups, commonly referred to as masters runners for runners over 40 years old.