Find out how long a MacBook has been running since its production
There's a Terminal command ac  


AC(8)                     BSD System Manager's Manual                  AC(8)

NAME
     ac -- display connect-time accounting

SYNOPSIS
     ac [-d] [-p] [-w file] [users ...]

DESCRIPTION
     A record of individual login and logout times are written to the system
     log by login(8) and launchd(8), respectively.  The program ac examines
     these records and writes the accumulated connect time (in decimal hours)
     for all logins to the standard output.

     Options available:

     -d      Display the connect times in 24 hour chunks.

     -p      Display individual user totals.

     -w file
             Read raw connect time data from file, instead of the system log. &gt;
     users ...
             Display totals for the given individuals only.

     If no arguments are given, ac displays the total amount of login time for
     all active accounts on the system.



It looks about right when I type it in my new machine, but my 9-year-old Mac Pro gives the same figure for its current user as total hours - which means it's lost track of my old account [still active on there but rarely used now] which as been in place since 2008. It just gives 0 hours.

It wouldn't be able to account for any hours the machine may have been on with no-one logged in.