My car seems to have difficulty starting from time to time, it is intermittent

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AS stated above, at times when I go to start my car (cold or warm), It seems to struggle to turn over and at times it just fires right up. This has been going on now for 6 months, Till 2 days ago I was at the store and when I got back inot my car and tried to star it it jsut made a grinding sound and slowly started to struggle more and more as if the battery was dead. I sort of pumped the accel a bit (but! that is pointless with all the electronics) And I let it sit 30 sec. tried to start it again it made that horrible slow grinding sound again, then all of a sudden it made a pop and the engine started right up. It was idling smoothly and as normal and drove home Onegin quiet/smooth ran normal. I just assumed it was the battery since it is the orig battery. I have had AAA come out they ran a diagnostics and found the battery , to mine and his amazement, was in grt shape fully charging 600A. The diagnostic checked the alternator and starter as well, The starter showed some declining in voltage below 12V and dropped (meter down to below 8 volts during start up. So! he said the problem is with the starter. Now here is my concern!, Even though this is a 2008 S60, which means 8 yrs old!. I only have 18,500 real/orig miles on this car I am the original owner and purchased new. I have had no issues in fact this has been one super nice trouble free car. Well up till this started!! :). Again when all this is going on the dashboard instruments, (check engine light doesn't light up except during starts of course) and on board messaging center give no warnings or alerts to a problem. Aren't starters typically have a life span of 100K and up , not 18K.. :(( I read it could be as minor as dirt that may have gotten into the connector (electrical) or a cracked connector or exposed wire, insulation is cut near the starter, or a flywheel issue, etc.. If a flywheel , well that raises even more!!! questions