Template:AgeFull
Copied from Wikipedia's For year month day template. The three parameters are "year", "month", and "day", all optional. Here's what happens if you supply a specific date:
"Oni is {{AgeFull|year=2001|month=1|day=28}} old"
yields
Oni is 25 years and 20 days old
whereas "Oni is {{AgeFull|year=2001|month=1}} old"
yields
Oni is 25 years and 0 or 1 month old
and "Oni is {{AgeFull|year=2001}} old"
yields
Oni is 24 or 25 years old
If you're content with just getting an age in whole years and you don't want the equivocating ("this or that"), use the simpler template Template:Age.