The most common belief in this category is also the most expensive one: that old means valuable.
What actually moves the number
Rarity within the series — mint mark, year, variety. Some recent issues are scarcer than some very old ones.
Strike and condition, described in the grading language this trade actually uses.
Originality. An uncleaned coin with honest toning is usually worth more than a brightened one.
For notes: serial, signature, series, and the condition of the paper, especially folds.
Why two of the same differ
One has been cleaned. A well-meant polish removes more value in an afternoon than any courier ever has, and it cannot be undone. How coin condition is described.
How to find out rather than guess
Identify the exact issue first — year, mint mark, variety — because that is what a comparable has to match. Then look at completed sales in the same grade.
Where you cannot grade confidently, say so and compare against a band rather than a number.
What age alone is worth
Very little. Large numbers of old coins survive in quantity, and a common coin in worn condition is a common coin. That is not a disappointing answer; it is the one that stops people paying for a valuation they did not need.
Questions people ask
Should I clean a coin before selling?
No. This is the clearest "no" in any of these guides.
Is a coin in a plastic holder worth more?
If it is graded by a service, the grade carries weight. A plain holder is storage, not evidence.
What about a whole collection?
Sell the identified pieces individually and the rest as lots. Burying a scarce coin in a bulk lot costs you.
More: pricing coins, selling coins.