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=29}} old"
yields
Oni is 24 years and 20 days old
whereas "Oni is {{AgeFull|year=2001|month=1}} old"
yields
Oni is 24 years and 0 or 1 month old
and "Oni is {{AgeFull|year=2001}} old"
yields
Oni is 23 or 24 years old
If you're content with just getting an age in whole years and you don't want the equivocating "X or Y years old", use the simpler template Template:Age.