controlling for age in ANCOVA in age-matched groups

Dear sci.stat.math,
I have two groups (a clinical population and an age and gender matched
control population).
The two groups have a large age-range.
The two groups are significantly different on a number of cognitive
measures.
These cognitive measures also vary substantially over the age-range.

Should I co-vary for age in the ANCOVA?
It seems to me the answer is 'yes', because age is a known source of
'error' variance, potentially overwhelming the effect of interest (the
cognitive differences between the groups).
However, I have also been told that one shouldn't control for age if
one has groups matched for age.
Scale Symmetry.

.

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