There is no direct way of finding the creation date of an index. But however if you look in sysobjects there is a creation date for Primary Key or Unique Constraints.Indexes associated with primary Primary Key or Unique Constraints creation date can be known.
check if there are any new DMVs (Dynamic Mgmt Views) in sql server 2008 which can help you in getting creationDate.
check this query
join sysobjects o ON o.id = i.id