Its not really how old the Topo Map is, it how accurate it was when it was made and how much has changed in the area since. Accurate Map with very few changed = Good Useful Topo. Still given a choice I would take a Topo updated last year in comparison to one made 30 years ago.
The only other MAJOR problem with an old Topo map is that the magnetic variation changes over time... Sometimes dramaticly!
The magnetic poles move around quite a bit...
Here is a very good link showing some rather EXTREME changes...
Magnetic Declination
http://www.compassdude.com/compass-declination.php
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