Everyone has answered, it’s the time it spends in the barrel that makes the flavor unique.
IMPORTANT THOUGH: once it’s bottled, the flavor won’t change. An 18-year Macallan that was bottled 5 years ago is still an 18-year Macallan. Not a 23-year Macallan.
Also, older doesn’t mean better. It’s just different. I’ve had some old scotches and the old ones aren’t always better.
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