Is it possible to enter junior tournament based on ATP/WTA rankings?

Question

Many players who are still eligible to play as juniors already enter enter senior tournaments. It is not uncommon to see, that some player plays qualifying at Grand Slam tournament and if they do not qualify, then they enter boys/girls competition.

It can happen that a player concentrates only on senior events, in which case they do not have points in the juniors ranking. But still such a player might want to enter at least some junior tournaments. For example, I think that junior grand slams are considered prestigious. If such player has enough points in senior ranking, but does not have point in junior rankings, can they enter the main draw? Or do they have to play qualifying?

I have noticed in Wikipedia article about tennis at the 2014 Summer Youth Olympics that one player entered based on being in Top 450 in the ATP ranking. However, Youth Olympic Games are kind of a special event, so it is possible that this was an exception and not a rule.

To summarize my question shortly:

Is it possible to enter junior grand slam tournament (or other junior tournament) based on ATP/WTA rankings (instead of junior rankings)?

Answer 1

Yes, It is possible to play in an ITF Junior event after having already obtained an ATP or WTA ranking. However, I believe it is your junior ranking that is taken into consideration before your ATP or WTA ranking. See the section titled "ITF Entry Regulations" here. Rule 46 of the ITF Junior Circuit Regulations also spells it out in a little more detail if you want to read about the "Criteria for Acceptances" in this PDF (page 14 of the PDF, or overall page 21)

Answer 2

To make this site more self-contained (and also to prevent possible link rot) I will copy here a part of a document cited in jamauss' answer:

Criteria for Acceptances

Singles

In determining acceptances of players for the Main Draw and Qualifying, the following acceptance method will be used:

• a) ITF Junior World Ranking of the Monday before the entry deadline. In a situation where two or more players are ranked equally, acceptance order will be drawn by lot.
• b) ATP/WTA rankings of the Monday before the entry deadline – Players ranked in the professional game ATP 550 or better or WTA 350 or better will always be accepted at the bottom of the Main Draw Acceptance List. Players ranked in the professional game ATP 750 or better or WTA 500 or better will always be accepted and remain at the bottom of the Qualifying Acceptance List.
• c) For all unranked entered players, the following acceptance method shall be followed firstly for vacancies in the Main Draw, if any, and then for vacancies in the Qualifying Acceptance List and Alternates List:
  i) – 50% of remaining places within the list will go to players from or resident in the country where the tournament is organised according to the national ranking submitted to the ITF by the host National Association. The remaining spots will be filled as follows:
  ii) - Players from, or resident in, other countries within the Region where the tournament is organised. The country will be drawn by lot and the top ranked player according to the national ranking who has not already been accepted will be accepted.
  iii) - Players from, or resident in, other countries within the Zone where the tournament is organised. The country will be drawn by lot as above. (This will include players from, or resident in, bordering countries which are not in the same region/zone e.g. Azerbaijan/Turkmenistan).
  iv) - Players from outside of the Zone where the tournament is organised. The country will be drawn by lot as above.
  v) - Any remaining players from or resident in the country where the tournament is organized.

For the other document I will provide at least Wayback Machine link.