First off: Chronicles of Darkness has its strengths and its weaknesses; simulating the concept of age is not one of its strengths.
For example, there's no Trait or even space on most character sheets that is intended to reflect age. There are some expanded sheets that do include it under description, but there are no passages describing, for instance, penalties to a dice pool. So, if a character has strength of 5, they can lift a coffin [1st Ed Storyteller's Screen] whether she's a 16 year old girl or a modern Jack Lalane. Storyteller fiat should help guide characters into reasonable traits, but mechanically, there's no difference.
However, the correlation between age and decrepitude can be modeled - Hurt Locker page 57 - 58 talks about how to model physically debilitating Conditions in a chronic or longtime fashion - for example, Deaf.
Flowers of Hell, page 34 details Conditions as part of a Pact under "Esoteric Pacts." The problem is, when computing the mechanical impact of a Pact, Mortals gain through the acquisition of Skills and Merits, not the alleviation or imposition of Conditions - this is also why FoH notes that conditions that are worthwhile to take [for the Demon] are very rare.
So, what we have is a Pact wherein the Demon mechanically can annul age-caused Conditions and alter the age of the pactbound; the Demon incorporates those Conditions or age alterations into themselves.
What are the mechanical effects for the Demon to take age-caused Conditions? Basically, Cover Experience. While Demon Debbie, since she has Eidetic Memory and can use Poker Face [FoH page 25] could act like she's deaf as part of her Cover as "deaf pianist", if she instead took the Deaf Condition into said Cover, she could generate Beats for said Cover.
What are the mechanical effects for the Demon to alter a Cover's age via a pact? None. Even the "Esoteric Pacts" section says
[Increasing age] to reduce the likelihood that she will be perceived as a physical threat.
Which sounds like it'd be a penalty for a medial-and-above Integrity character to initiate violence against the Demon?
For the Mortal; Conditions caused by age being alleviated would be the mechanics from their end.
As far as mechanics for altering their age - there's not really an effect - other than the presumably increased lifespan.
And, if the Pact was just between the two of them, it'd be smooth sailing, but it's not - the Pact changes reality.
It's like a hurricane: The Demon and pactbound stand in the eye - they are aware of what has changed with perfect clarity. Nearby, family, bosses, etc. are affected - they don't remember life before the Pact - their sky is covered in the hurricane. But far enough out, and the hurricane ebbs away. And that's where the big danger in terms of age-shifting via pacts come in.
If Todd shows up to his high school 50th reunion and he's pushing 30, people are going to raise their eyebrows. They remember him from class, but he's half their age! And when these eyebrows turn into questions, that's when the 70 year old Cover starts eroding. A "classmate you never talked to" is not going to be "close enough", techgnostically speaking, to the pactbound to get swept up in the Pact's effect, but he knows enough to be able to pry.
Vulnerability to Compromise cuts the other way, too: if a Demon "gives" seven years to a 14 year old desperate to gain independence, his previously same-age girlfriend's father might have a rude awakening. Matters could be further complicated if the boy's parents had only met 18 years ago - that sounds a Compromise roll penalized to a chance die, if it were my call. Or perhaps, the Pact changed the parents' memories - meeting 3 years prior, no longer waiting until marriage to have sex, etc - all kinds of large, GM-perceptible ripples.
One final extreme: a mortal says "make me 20 again" - except he has a 25 year old daughter. What happens?
Here, we can look into Heirs of Hell, page 19: what happens when a Demon has a pregnant Cover? Different alternative Conditions are described to give the troupe flexability; the best ones to adapt would be Locked Cover (the man can't make the Pact), Irreconcilable Paradox (the child is nixed from existence), and Flexible Parenthood (the child gets someone else as the father).
In short, the mechanical benefits are slim and circumstantial; the threat of Compromise is very high unless the pactbound was a true loner; and the ripples from the Pact could be quite observable.
In short, it's probably safest to just fake your Cover's death while leaving everything to another Cover in your will; and if you're making a Pact with a Demon from a nursing home wheelchair, make sure the Demon specifies both age and restoration of mobility, or you would end up with youth in a wheelchair.
For other Supernaturals, remember that dead flesh doesn't "age" - you can't load 200+ years off a Vampire or Mummy (don't know why one would try)
And if the pactbound is a Sin Eater (and possibly Mage) getting a "new lease on life" from their deathbed, it'd prolly be reasonable to take a hit on Synergy (/Wisdom) for perverting the "natural path of Death" (/a fairly hubris act)