The first quarter of this century is over at the end of 2025.
2001 was the first year in this century. 2025 is the 25th year in this century. 2100 will be the 100th - and last - year in this century.
(1 was the first year in the first century, 100 was the 100th - and last - year in the first century. That's why every subsequent century starts on xx1 or xxx1 as well)