1) You don't have to. Different parts can have been made at different times. So you can get a phone that was put together in January 2015 with parts that were made in July 2014. What does the 2015 date tell you? Not much. How do you find out when each part was made? Even if Samsung allowed you access to their databases, hiring someone to get the data on all the parts in your phone would cost about as much as a few crates of S5s.
2) You need a site with Samsung's databases (which is about as easy to find as flying elephants - IOW, the only way would be corporate espionage). Samsung knows the assembly date of every IMEI, but anyone working for Samsung who's found releasing information like that is probably going to ... well, it's South Korea, not North Korea, but even so.