Coconut Battery must give the manufactured age because I've had my MBP for about 2 weeks now, but Coconut Battery tells me its 8 weeks old.
Why don't you use coconutID? It will tell you when and where it was made.
😉
That's not what the OP is looking for. He wants to know how long his computer has been used for.
So I guess none of the responses so far are much help to the OP.
To answer the OP, there is actually a very straight forward solution, which is so obvious, people forget about it!!
When you first turn on your Mac, you need to configure the first user account. If you navigate to the Users folder, and highlight the user account folder and 'get info', it will tell you when that folder was created...