OK, mortar board out again!
Latin Senex, an old man.
Was originally used to denote seniority (from the same stem) and respect. Has become derogatory because it is now associated with weakening mental capacity.
Latin Juvenis, a young man.
Originally neutral, denoting someone under the age of maturity. Has also become derogatory because of associations with juvenile court, juvenile behaviour, etc.
They'll both probably retain their derogatory interpretations, language doesn't usually regress.
End of lecture!
PS Senator also come from senex!