When can you start to file your taxes?

You start Jan 1 of the year you are paying taxes for in the sense that you save all receipts for things that might turn out to be deductible. Like a minor medical expense might be deductible if you have a surgery or two later in the year.

You pay estimated quarterly taxes if you don't have sufficient withholding. Like if you are self-employed.

The above is actually entirely correct...you start getting the information and paying your taxes through the year. You file them after collecting all the information at the end of the year.

THIS QUESTION HAS BEEN ASKED AND ANSWERED MANY, MANY TIMES AND IS PART OF THE DATABASE. (As a courtesy one is linked here as a "related question").

CORRECTLY USING THE SYSTEM WILL PROVIDE THE ANSWER WITHOUT ANY DELAY, (or requiring inconvenience or action by anyone else).