I might be repeating what’s been said but one indicator to me that a wine is too young to be considered ‘ready to drink’ is sensing direct, almost unmediated fruit and herb notes. Another is just not sensing much of anything. Often it’s a combination of the two. A too young wine can still be worth drinking for various reasons, not least of which is to try to assess what it might become. But it shouldn’t be evaluated as presenting in its fullness.
By the way, here’s a prior conversation on the subject of the sensory experience of wine age: https://www.wineberserkers.com/t/describing-wine-primary-secondary-and-tertiary-notes/306591/3. There’s quite a bit in this conversation – links to articles on age (primary, secondary, tertiary) notes and more.