It isn't stored in
sys.dm_exec_cached_plans, nor is it buried anywhere in the plan XML that I can find. There is useful information in other DMVs however.
For stored procedures we can get the time a plan was cached from
p.name AS [SP Name]
sys.procedures p WITH (NOLOCK)
sys.dm_exec_procedure_stats ps WITH (NOLOCK)
ON p.[object_id] = ps.[object_id]
ps.database_id = DB_ID()
For ad-hoc queries, there is the creation time in
st.[text] AS [QueryText]
sys.dm_exec_cached_plans cp WITH (NOLOCK)
sys.dm_exec_query_stats qs WITH (NOLOCK)
ON qs.plan_handle = cp.plan_handle
cp.objtype = N'Adhoc'
According to @SqlKiwi... the change from creation_time to cached_time was just because procedures and triggers were added in 2008 and the opportunity was taken to choose a more descriptive name. The created/cached time reflects the last compilation, not the creation time of the original plan.