Contrary to popular belief, your age does not determine how wise you are. This flawed notion doesn't measure up to reality.
This fallacy is based on the assumption that the older you are, the more experience you have, therefore equating to more wisdom. It's a narrow-minded perspective and provides people with self-entitlement to assume they're right merely because they're older than you, regardless of the opposition's merit.
Experience isn't always diverse
Consider the following situation: there's two people of both differing ages and wisdom.
The first is thirty years old, has solo-travelled the world for 4 years, has diverse interests, loves trying new things, and is both objective and open-minded.
The second person is sixty years old, has spent the majority of their life doing the same things, is repelled by the novel, and denies anything contrary to their worldview.
Who do you think is wiser? I have no doubt it'd be the first person. This is because wisdom is gained from a large range of experiences, not from doing the same things for a very long time.
A wide range of experiences enables you to draw unique and better-informed conclusions—essentially the definition of wisdom. On the contrary, possessing only a narrow pool of experience limits the wisdom you can gain.
Experience doesn't always translate to wisdom
Following the previous example, even a diverse range of experience doesn't necessarily result in wisdom. Insight is gained through proper assessment of your experiences, therefore without this your wisdom is limited.
Two requisites for deriving wisdom from experience are an open mind and the ability to objectively and accurately assess your experiences. These go hand-in-hand and enable a person to reach the correct conclusions—the truth—without bias due to bogus external influences such as the status quo.
Wisdom can be derived without first-hand experience
You don't necessarily need to experience something first-hand in order to gain insight from it. By second-hand experience, I mean gaining insights from other peoples' experiences through other mediums, such as books.
For instance, you could read many books on a diverse range of topics, each providing a unique perspective. This wide range of perspectives enables you to draw more accurate conclusions, thus effectively translating to wisdom.
For this reason someone who has plenty of second-hand experience can potentially be wiser than someone with little, as the latter is limited to their narrow worldview.
Open-mindedness is rare
The fact is humans are hard-wired to conform. This has significant implications on wisdom, because as mentioned, open-mindedness is required to draw the correct insights out of your experiences.
There's substantial evidence proving this, such as a famous study conducted in the 1950s to assess the number of people who conform under peer pressure. The results are stark: about 75% of people tested conformed under peer pressure even when their action was wrong, versus the control group who conformed 1% of the time (those with no peer pressure).
Or consider neuroplasticity, which argues our neural pathways begin to solidify around age 25. This makes it difficult for one to alter their worldview and opinions once they've been solidified—especially once reinforced throughout their life (such as with old people).
For this reason it's plausible to argue age provides a scope of wisdom defined by the diversity of the person's experiences and the integrity of their assessment of it. Therefore someone who is old may be wise about a few things, but extremely close-minded and ignorant about anything outside of that.
Age does not equate to wisdom. It's a popular fallacy arguably used by people to validate themselves, invalidate those who are younger, and reject information outside their worldview regardless of its merit.