It has to do with the tournament trying to keep competitive balanced opponents for as long as possible throughout the duration of the tournament. It's the same reason why the tournament distributes seeded players evenly into the draw to begin with so that they avoid high-profile matches until the later rounds of the tournament.
Related: See my answer about ...
ATP Tennis or The Association of Tennis professionals is the governing body of tennis in the men's circuit of professional tennis. It consists of all the tennis tournaments organized by the Association of Tennis Professionals or ATP. The organization is affiliated to the International Tennis Federation or the ITF, the international governing body of tennis. ...
ATP World Tour / Challenger
According to section 7 of the ATP Rulebook, the following restrictions apply for male players:
A. Gender / Age Limitation
The ATP will use the age of a player on the first (1st) day of the
qualifying competition. Male players age sixteen (16) or older are not
limited in the number of tournaments they ...
WTA/ITF - Women Competitions
For more details you can look into
2015 WTA Official Rulebook (Internet Archive). Briefly summarized it says in the section XV. Age eligibility and player development that:
Players under 14 cannot enter tournaments on WTA or ITF Women's Circuit.
Above 14 players can play in Fed Cup.
14 years old player can play at 8 ...
The main differences are which organizations organize and operate the tournaments, how much prize money and how many points are up for grabs, and the rankings of the players in the draw.
Futures: Run by ITF, Prize money of 10-15k, ranking points from 1 point to 35, players ranked anywhere from 2000+ in the world down to about 150 or so.
Challengers: Run by ...