This is possible, but quite complex to do with fields, as you will see from
the example shown below. The better plan is to see Macropod's master work on
the subject at www.wopr.com/cgi-bin/w3t/showthreaded.pl?Number=249902 Copy
and edit the example to your requirements substituting your mergefield for
the ask and ref fields.

{QUOTE{SET by {MERGEFIELD Birthday \@ yyyy}}{SET bm {MERGEFIELD Birthday \@
M}}{SET bd {MERGEFIELD Birthday \@ d}}{SET yy {DATE \@ yyyy}}{SET mm {DATE
\@ M}}{SET dd {DATE \@ d}}{SET
md{=IF((mm=2),28+(mm=2)*((MOD(yy,4)=0)+(MOD(yy,400)=0)-(MOD(yy,100)=0)),31-((mm=4)+(mm=6)+(mm=9)+(mm=11)))}}{Set
Years{=yy-by-(mm<bm)-(mm=bm)*(dd<bd)}}{Set
Months{=MOD(12+mm-bm-(dd<bd),12)}}{Set Days{=MOD(md+dd-bd,md) \# 0}}"Age is
{Years} Year{IF{Years}= 1 "" s}, {Months} Month{IF{Months}= 1 "" s} and
{Days} Day{IF{Days}= 1 "" s}."}