The Windows NT logs are the primary place to find when the account was first used - the eventlog comes with a find/filter option. However, it may have rolled depending on the configuration.
Check the SQL logs too - if the last restart was some time ago, it might give you a clue as to when.
If SQL Server is running under a domain account, it would be worth checking AD - many organisations put reference numbers against new accounts etc. Also have a trawl of any incident/request management system you might have.