Add them like you add anything else. Dates are really just floating point numbers.

First, calculate the ages of each i.e. convert it to total days: 365*Years+30*Months+days

This won't give exact numbers since not all years are 365 and not all months are 30 days. If you want, 365.25 for years and 30.4 for months.

Better yet, you can calculate the number directly from their birth date: =now()-Birthdate
For this, it's probably best that you enter the birth date in another cell and reference that cell in the formula e.g. if the birthdate is in A1: =now()-A1

Now just add 'em all up. If you want, you can format this number to display any number of ways e.g. yy,m,d