For indexes that are constraints, then see marc_s’ answer
For other indexes, you’d have to use STATS_DATE to get the creation time of the associated index (every index has statistics on it)
Something like (not tested)
SELECT STATS_DATE(OBJECT_ID('MyTable'),
(SELECT index_id FROM sys.indexes WHERE name="Index_Name"))
This relies on the sys.indexes to sys.stats links
Edit: there is no way to find out as far as anyone can find out. Sorry.