SQL Server uses LRU (Least Recently Used) algorithm to keep track of pages that are aged in the buffer pool. It increments a counter every time a page is referenced and decrements the counter every time the lazy writer process sweeps the page. Any worker thread will check the memory status of Buffer Pool to make sure that there are healthy numbers of free pages to honor incoming new requests. 

Note that: SQL Server will always keep a minimum number of free pages on the free list, so that it can serve incoming requests without any significant delay. The amount of free space is calculated based on Buffer Pool and amount of incoming requests.