I know character descriptions happen when a character is first introduced but what about when you are dealing will a character at different ages?

Right now, my story starts out like this....

Briefly see characters in present day.  (This is where I placed the character descriptions)
V.O. from a character brings us into a flashback.
We have a short sequence of scenes in the flashback.
V.O. brings us back to present day.

I refer to the characters as YOUNG JOE etc. in the flashback but didn't include character descriptions (other than age) because it seemed redundant for such a short scene.  I'd rather have the descriptions represent the character in the present day since that is where the rest of the story takes place.

However, I'm now considering getting rid of the V.O.  I don't like how the story is being forced from that characters perspective.  So now I'm thinking for the flow of things, to make the flashback the first scene.  So it simply would be...

Flashback
Present Day

So my question is do I need to write the character description for both Younger and Present day versions of the character?

or

Can I have the character description for the Present Day and assume a casting agent will realize the characters in the flashback are just younger versions?

or

Do I simply move the character description to the Flashback even though the description I currently have more reflex the character in the Present Day?

I have a feeling I'm probably overthinking this.  I guess maybe my hang up is that I'm looking at the younger versions of the characters as "different" characters than their present day counterparts.  

It depends.  Unless Young Joe is drastically different from Modern Joe, I wouldn't bother.  Like if Modern Joe is missing a leg, but Young Joe isn't, you'd want to indicate that.  Or if Modern Joe is a sulking, brooding depressive but Young Joe was cheerful and hopeful (or vice versa), you'd want to point that out.  It really depends on the purpose of the flashback, and what kind if information you're attempting to reveal about the character.

I'll echo Lon's response