Junior tennis refers to tennis games where the participants are aged 18 and under. Eligibility to compete is not based on age, but year of birth: as a result, some players must move out of juniors soon after their 18th birthday, while others can play juniors until they are nearly 19. Some players who qualify as "junior tennis" players also play in main adult tours, though forms signed by their parent or guardian are required for this.
Could somebody explain the rules about eligibility for junior tournaments in more detail? For example, as now is 2016, are players born in 1998 and later the ones who are allowed to compete this year in junior competitions?
It would also be nice to have some source for this. (As the Wikipedia article I linked to lacks references.)