to calculate the average for a single age field for all the found records.
Use a summary field, not a calculation field.
Thanks! any idea how to count the field ONLY if someone answers yes OR no only (ie ignore one or the other)?
You'll need to add a calculation field (or two) and count (or sum) it. If "yes" and "no" are the ONLY options, you could use 1 and 0 instead - this way you can use Sum and Count directly.
Another option is to produce a report, subsummarized by the field.