How to use a field code in MS Word to calculate age?

I am not a programmer so please be gentle!

In an MS Word document I need to calculate age in years from birthdate.

I’ve done a lot of searching and seem to think this may work:

{QUOTE{SET by {BirthDate @ yyyy}}
{SET bm {BirthDate @ M}}
{SET bd {BirthDate @ d}}
{SET yy {DATE @ yyyy}}
{SET mm {DATE @ M}}
{SET dd {DATE @ d}}
{Set Years{=yy-by-(mm<bm)-(mm=bm)*(dd<bd) # 0}}

My problem is that the above seems to refer to a field code
and I have no idea how to work with this.

Please lend a guiding hand with step-by-step instructions
for how to use the field code and the steps above to calculate
the age in years from a birthdate.

Thanks so much!