Learn Farsi: شکوفه‌ی جوانی سریع از بین می‌رود.

This is an Ezafe-ye Tashbihi (metaphorical Ezafe). While it translates to the English phrase "the blossom of youth", it doesn't mean youth possesses a blossom. Instead, it equates the two nouns directly: youth is a blossom. Linking two nouns with an Ezafe is a very common way to create a structural metaphor in Farsi.

Why is there a ی at the end of شکوفه‌ی?

The word شکوفه (blossom) ends in a silent heh, which makes an "e" sound. When you add the Ezafe to a word ending in a vowel, a consonant "y" is inserted to bridge the gap, making a "-ye" sound. In modern Persian writing, this is usually spelled by adding an independent ی after the silent heh, though you might also occasionally see it written with a hamza over the heh as شکوفۀ.

What is the literal meaning of the verb از بین می‌رود?

The phrase literally translates to "goes from between" (از means from, بین means between, and می‌رود means goes). As a fixed compound verb (از بین رفتن), it is the standard way to say "to perish", "to be destroyed", or "to disappear".

سریع usually means "fast" or "quick" as an adjective. How does it become an adverb here?

In Farsi, you do not typically need to add a suffix (like "-ly" in English) to turn an adjective into an adverb. Adjectives like سریع (quick) are simply placed in the sentence and used exactly as they are to mean "quickly".