When is a job applicant considered too young or too old for the position applied for?

Only when the person doing the considering is prejudiced, and values their own bigotry more than the needs of the company, the law (in many countries) and morality.

if the candidate is above 18 years old and under 65 years old, mentally and physically fit for the requirements of the job then should be given a chance to compete. i meet UN volunteers in their 72 of age.

if he comes under child  labor then too young if working after retirement age then he is too old.