1) "what have you done lately?" This is an old Broadway audition adage about not relying on great performances from years ago but what you are currently like
2) Most of the applicant pool will have nice freshly dated letters from this year and that is what the adcoms will see.
3) While many schools have loosened their types of letters, I would say it is an expectation by an adcom evaluator to see 2 science and 1 non science academic letter. Exception to this may be for nontrads when a letter from a work supervisor is expected. Additionally, PI letters are expected. Letters from other sources, such as volunteer coordinators or doctors you shadowed are at best secondary and at worst seem like fluff
4) At the same time in loosening letters, there has been a marked increase in schools becoming stricter in the number of letters. This is solely due to increase in application
numbers and workload throughout the admissions process at a school. Many schools are now simply marking a file a complete and moving to evaluation queue when the max letter requirement number is met. So if you assign 6 letters and the schools max is 4, the first 4 letters that show up, get in the file, and the other 2 are essentially unused.
5) whoever write that letter for you can be contacted and asked to update.
6) After all this, you can used the dated letter. How much lessened impact it has from being older and non-academic will vary across schools and individual adcom members