Exactly.
To better illustrate, imagine you start working at 15 (for ease of calculations) at a yearly income of $20,000. At 20, your income increases to $40,00. At 30, your income increases to $60,000 and stays there until retirement.
If you work until 65, the 35 years of highest salary will be all at the $60,000 level.
If you work until 55, the 35 years of highest salary will be 10 at $40,000 and 20 at $60,000.
The latter will be lower than the former. Even if you don't collect until 70, the calculation will be based on those years worked.
To further illustrate, suppose you left your "high paying" $60,000/yr job at 55, and took a new job that only payed $30,000/yr. You work until you're 70. Your calculation will be based on the 10 years at $40,000 and 20 years at $60,000.
Now, if your pay has been the same for the majority of your career OR you made enough from early on to have always contributed the max amount, your benefits may not be affected by your "stop work" point. But, for many, their early work years were at a significantly lower level than their later work life and they may not have 30 years at the higher level by the time they have to stop working.
