It's difficult to do further calculations with the formula you quoted because that returns a text result so to calculate the average age it's best to calculate the average date of birth, e.g. if you have dates of birth in J252:J300 then in J302 use the formula

=AVERAGE(J252:J300)

then run your original formula on this average date of birth to give the average age, i.e.

=DATEDIF(J302,NOW(),"y") & " years, " & DATEDIF(J302,NOW(),"ym") & " months, " & DATEDIF(J302,NOW(),"md") & " days"