Details (numbers as of Sep 2015):
To prevent tag fragmentation, I think that amenity=youth_centre should be deprecated, and tagged
amenity=community_centre is used 29307 times, and rendered with icon and name on the general map.
The wiki definition of community_centre includes youth centres, among other locations targeted at specific age groups. Thus it makes sense to specify them with a sub-tag.
amenity=youth_centre is only listed as a "tag being used" in the wiki, and used 759 times. An overpass query shows that these uses focus mainly on Germany and England (UK).
Sub-tagging with community_centre=* was already proposed on the talk page in 2010, though the key is only used 71 times so far, for describing the target groups, such as =children or =elder
An alternative would be to reserve the community_centre=* for the type of the facility (e.g. =meeting_room, assembly_hall), and follow the scheme introduced in social_facility:for=*, that means defining community_centre:for=*, with mostly the same values. The social_facility:for scheme is well supported with 8534 uses. Example: