My question from here on is how old is the scripture expounding the birth of Dorje Shugden as the son of Ganapati and the Queen of the World?
The verses quoted in the original post are part of the praise to Dorje Shugden called
Infinte Aeons, ascribed to Dagpo Kelsang Khedrup, who lived two generations before Pabongkha Rinpoche, as you can see from
http://dorjeshugdenhistory.org/among-shugden-texts-1800a.html. Therefore, the scripture should be some 160 years old.
I fear that it may be seen as some kind of fairy tale by non-believers.
What makes a scripture dealing with very hidden topics more or less credible is not its age, but rather the credibility of its author. A credible author is an author who makes no mistake in relation to slightly hidden topics, which we can check by ourselves, such as emptiness. Examples are Buddha Shakyamuni, Nagarjuna, Tsongkhapa, and Pabongkha Rinpoche.
All truth comes from scriptures
And many lies as well. Therefore, when it comes to very hidden topics, one should rely only on scriptures written or spoken by reliable teachers.
so it would be good to have authenticated source quoted.
The full text of
Infinite Aeons can be found starting on page 138 of Trijang Rinpoche's
Music Delighting the Ocean of Protectors, which can be seen here:
http://www.dorjeshugden.com/articles/musicdelighting.pdf. Trijang Rinpoche's commentary on the verse quoted in the original post starts on page 100 of the same text.