Answer
The contamination would make the samples appear younger.
Work Step by Step
In old biological samples that contain decaying C-14, the smaller-than-current ratio of C-14/C-12 tells us how long ago they stopped taking in fresh C-14. Contamination from younger materials will increase the ratio of C-14/C-12, making the sample appear younger.
This contamination especially affects older samples, because they are the ones that have the least C-14. Any contamination will form a greater fraction of the C-14 that was already present in the sample.
