"Themselves" is a reflexive pronoun and is primarily used when the subject of the sentence is also the recipient of the action: "He tripped and cut himself with the knife he was holding." Hence, although many writers these days would not hesitate to write "younger versions of themselves," "younger versions of them" is actually correct. If you are a prescriptivist, go with the latter; if a relativist, the former.
"Theirselves" is not standard English and is considered ignorant or uneducated. You could, however, quite correctly write a phrase such as "versions of their younger selves."
As Fowler repeatedly points out, in cases like this one the problem usually runs deeper than is apparent. (The use of the word "reflections" to avoid repeating "shadows" is another symptom.) Recasting the sentence is usually a better cure, in Fowler's view. For example, "But the men's shadows are younger versions of them, not real shadows."