First things first:
As far as Demons not aging goes, pages 118 & 182 of DtD state that Covers age, even when not in use.
However, since demons can have Boltholes, wherein time does not pass, it is possible for a demon to "go to ground" for a decade and thus have a cover that people think is 10 years too young for the person they remember.
But to answer the question at hand:
Flowers of Hell, the player's guide, says on page 34:
"... the demon could buy [through a Pact] old age for one of her Covers to reduce the likelihood that she will be perceived as a physical threat."
Thus, it does seem that age is property of someone's life a Demon can obtain through a pact.
Edit: There is a second half to your question: "also without a break of Cover"?
All Cover derived from Pacts is risky; all such gains are vulnerable to investigation. For example, taking someone's wife is going to be fairly safe if few people would question it; this is why few Demons would consider "Pacting into" a celebrity marriage - such Cover gained would be gone in an instant.
As far as altering a cover's age, consider that many people have annual forms to fill out (likes taxes, car registrations, etc) that require a birthdate. Some agencies might have cause to investigate if an applicant's birth year changes, again, thus invalidating the dots gained through the Pact (at best.)
Naturally, the larger the change in age, the more risky a gain it is; if Demon Debra takes the youth of teenager Tim, he might then be older than his parent's marriage. The mechanisms of Cover would try to edit those memories - which sounds like a sure-fire way to cause a noticable ripple in reality - precisely what Cover is meant to avoid.