The MSAR lists the "Oldest MCAT Considered" under the "Application Process and Requirements" for each school (at least it did last year). There's no stated limit on LORs, but within the past 2-3 years should be fine if you're a traditional student (assuming some of the letters are pretty recent). Non-traditional students may want to add "supplemental" letters beyond the required letters (if the school allows them) to help explain what they've accomplished along the way--these can be older, if they're really relevant (e.g., they conduct research at that med school, or from your supervisor where you worked for 10 years who might have a reasonable opinion on what you'd be like as a doctor). Since they're additional LORs, the adcom can just ignore them if they like, but sometimes they help round out the picture for adcoms that take the time to read them.