"He is older than her."
Translation:Il est plus vieux qu'elle.
Most of the time we don't use "ancien" (as an adjective) for people, at least not for the meaning "old" (age).
We can use it however to mean "old" (ex-) for someone who lost his position (or anything like that):
- l'ancien entraîneur de l'équipe
We can also use it to mean "old" (age) as a noun:
- les anciens de ce village
Because adjectives are used with what we call in French "un verbe d'état" (a stative verb).
Here are all the stative verbs in French :
"être, devenir, paraître, sembler, demeurer, rester, avoir l'air, passer pour"
When adjectives are used with such verbs they're called "adjectif attribut".
Of course they can be used without a verb as well, then they're either "adjectif épithète" (not separated from the noun) or "adjectif apposé" (separated from the noun by a comma).
Watch out because it seems that English names for these categories are misleading. For example it seems that "adjectif épithète" is called "attributive adjective" in English.