I take it that's a reference to (what is translated as) this phrase, which appears in several suttas including for example the "Assu Sutta: Tears (SN 15.3)":
From an inconstruable beginning comes transmigration. A beginning point is not evident, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries — enough to become disenchanted with all fabricated things, enough to become dispassionate, enough to be released.
I won't answer your literal questions but here are a few comments.
Buddhism teaches that conditioned things are impermanent ... that anything which begins because of some cause (i.e. because the conditions for its existing have come together) will have an end when those conditions no longer apply.
So, it's interesting to know the cause (or beginning) of suffering: because knowing how or when it begins is a hint as to when or how it end (for example if a fire starts when there is heat and fuel, it ends when there's no heat or no fuel, or when heat is detached from fuel).
But samsara doesn't seem to have a beginning, doesn't have a "construable" beginning (and may continue endlessly). Even so it may be possible to escape it, to find release: by becoming dispassionate and disenchanted with fabricated things (also known as "conditioned things").
The Buddha could see his (and other people's) past existences, so the question ("how did it all begin?") was likely to arise (and people, maybe your physicists for example, still ask themselves that to this day).
But this ("How did the world begin? Did the world begin?") belongs to the category of unanswered questions.
When I was young I used to fight with my brother, and when my mum would come then we'd say, "He started it!" -- "No, he started it!", and she'd tell us, "It doesn't matter who started it, you both have to stop."
See also "a thicket of views" on this page which helps to explain why unanswered questions are unanswered.
See also for example the first few lines of the Dhammapada (including this verse), which suggest it doesn't matter whether you have a reason for fighting, what matters is whether you hold (attach to) that view.
Anyway, saying that the beginning is "inconstruable" is a way of answering (or not answering) the question, of closing that line of investigation.
The decision of what to investigate, of what to teach as dhamma, is a question of soteriology (see also "Simsapa Sutta: The Simsapa Leaves (SN 56.31)").
Finally, you say, "our universe is roughly about 13.8 billion years old".
I think that the antique cosmology assumed that the "world" is cyclic, subject to expansion and contraction. Perhaps there was another universe before this one, and another one after? It's inconstruable...
I want to say though that I don't think that Buddhism and Physics (including both astrophysics and quantum physics) are describing the same phenomena. I think that Buddhism is primarily a description, not of "the world" but of our perception of the world, of (our) consciousness, of man's relating to the world (including ethics and wisdom, etc.).
Anyway I think this description (i.e. that its beginning is inconstruable) isn't exactly being applied to "our universe": in the original context it's a description of "transmigration" i.e. of samsara.
In a comment, Samana Johann pointed out that the Pali phrase is:
Anamataggoyaṃ bhikkhave saṃsāro.
Here are dictionary definitions:
Definitions for anamatagga
New Concise Pali English Dictionary
without beginning (or end, generally of saṃsāra).
Concise Pali English Dictionary
one whose beginning is unknown.
PTS Pali English Dictionary
epithet of Saṃsāra “whose beginning and end are alike unthinkable”, i.e., without beginning or end. Found in two passages of the Canon SN.ii.178, SN.ii.187 sq. = SN.iii.149, SN.iii.151 = SN.v.226, SN.v.441 (quoted Kv.29, called Anamatagga-pariyāya at Dhp-a.ii.268) and Thig.495, Thig.6. Later references are Cnd.664; Pv-a.166; Dhp-a.i.11; Dhp-a.ii.13, Dhp-a.ii.32; Sdhp.505. [Cp. anāmata and amatagga and cp. the English idiom “world without end” The meaning can best be seen, not from the derivation (which is uncertain), but from the examples quoted above from the Saṃyutta. According to the Yoga, on the contrary (see e.g. , Woods, Yoga-system of Patañjali, 119) it is a possible, and indeed a necessary quality of the Yogī, to understand the beginning and end of Saṃsāra].
ana (= a neg.) + mata (fr. man) + aggā (pl.). So Dhammapāla (avidit-agga Thag-a.289); Nāṇakitti in Ṭīkā on Dhs-a.11; Trenckner, Notes 64; Oldenberg Vin. Texts ii.114. Childers takes it as an + amata agga, and Jacobi (Erzähl. 33 and 89) and Pischel (Gram. § 251) as a + namat (fr. nam) + agga. It is Sanskritized at Divy.197 by anavarāgra, doubtless by some mistake Weber, Ind. Str. iii.150 suggests an + āmrta, which does not suit the context at all
I infer from the PTS definition, that:
The derivation/etymology of the word is uncertain, people understand it from the examples (context) in which it is used.
It can be contrasted with the Yoga Sutras of Patanjali, which (according to the PTS) say that it's possible and necessary "to understand the beginning and end of Saṃsāra", and which (according to Wikipedia) is about the same time as the Buddha (400 BC).
I think that scholars find that there are various little bits of Dhamma which contrast other Dhammas (e.g. Brahminism, Jainism), as if the clarify sometimes ways in which Buddhist Dhamma differs from other dhammas.