Given that Clark and Barry are the "originals" (in the general sense), there's a difference.
Young Wally is too white and generic; adult Wally is both too white and makes Barry appear too old.
Batman can get away with it because he is DC's most popular character and can deal with any hits from being "aged" too much by having too many sidekicks - and even then they do squeeze them together into a much shorter timeframe.. Flash isn't in the same category.