Old football cards worth money are a smaller group than the phrase suggests, and the reason is the same in every sport: age lowers supply and it also lowers the number of sharp survivors. Those two pull in opposite directions on price, and which one wins depends on the card.
Smaller runs, rougher survivors
Football cards printed before the 1980s came from modest runs and were handled by children rather than stored by collectors. So there are fewer of them and a low proportion are clean. That combination is what makes condition swing the price so hard at that age: an average example and a sharp one can differ by a multiple rather than a margin.
Where the demand concentrates
Football collecting is organised around quarterbacks and around first-year cards more than any other position or issue. An old rookie card of a quarterback whose career mattered sits at the intersection of small supply and concentrated demand, which is where the money in old football cards actually is. Almost everything else from the same year is common by comparison.
The line to draw
Cards from before about 1980 are worth checking individually. Cards from the late 1980s and early 1990s generally are not, because production had risen sharply and collectors had begun storing everything carefully. That is a filter for where to start rather than a rule about value, and it saves a great deal of time on a large box.
Then price the survivors
Five completed sales of the exact card in the condition yours is actually in, through the card value checker. For older football cards be particularly careful about condition matching: the difference between the sales you find and the card you hold is usually condition rather than the market.
Questions people ask about old football cards worth money
Are 1970s football cards valuable?
Some are, particularly rookie cards of players who went on to matter and any card in unusually sharp condition. Common cards from that decade in average condition are generally not.
Do old team sets sell?
Slowly, and usually for less than the sum of the few cards inside them that people actually want.
How do I judge condition on a card that old?
Corners and surface first, then centring. Expect wear: the question is how much, relative to the other surviving examples you can see in completed sales.
Is it worth grading an old football card?
Where it is scarce and sharp, often. Run the cost and uplift calculators, and use the price at the grade you expect rather than the one you hope for.