When Does A New Century Begin For A Numismatist?

I'm in the "01" starts the century and "00" ends it camp.

A new century begins on the fist day of the first month of the first year of the century. thus it begins on 01/01/01. 001-100 fist century. 101-200 second century and so on

On his/her 100th birthday.

The first century is obviously 1-100 (and yes, there are coins from it, even dated ones, though naturally those dates are in non-AD calendars).
That means the second century is 101-200, the third is 201-300, and so on, up to the 21st century being 2001-2100.

As for the 2000s, to me this refers to the decade that ended in 2009 (similarly the 1900s were 1900-1909, and so on).

I like DBM's idea, incidentally.

whats a year among a 100
my 20th century type set goes from 1900-to 2000