Who started the SQL Server Agent? How to find out who started the 'SQL Server Agent' in Microsoft SQL Server 2012 (SP3-CU1) (KB3123299) - 11.0.6518.0 (X64)
Windows Applicationlog ID 102: SQLServerAgent service successfully stopped.
Windows Systemlog ID 7036: SQLServerAgent service successfully stopped.
SQL Server Logs: Event ID: [098] SQLServerAgent service successfully stopped.

It doesn't show who did it. I think you need to configure auditing or tracing for this. For tracing there is a Security Event called Audit Server Starts and Stops.