John, I presume this 15 years is from your typical stock head to a middle-aged man? There's a bit of a difference between how you'd go from 20 to 35 and 40 to 55
There are two broad approaches I can think of here. The first is additive and the second is subtractive. Many men 'fill out' as they approach middle age (I speak from experience) and their faces become fuller and less defined than in their 20s. If you want to go this route I would fill out the cheeks especially and also the sides of the head - the cheekbones and the temple area - you deposit fat quite widely on the head, as you can see from CAT scans. If the original head has quite flat upper cheeks under the eyes you can add a bit of a bag here too. Remember to target the neck and underside of the chin: the neck will tend to become fuller and lose definition and there is usually a change from the taut horizontal line from chin to throat to at least a slight downward angle, if not a definite fullness visible from the front.
For the subtractive approach, for someone you imagine has kept fit and health and lost his 'puppy fat' (like my current avatar in actual fact) I would hollow the cheeks in the area just under the cheekbones. You could also scrape down the temple a bit to define the area where the temporal muscle attaches from the jaw. In addition to just using a sharp #10 blade in the X-Acto I like to use a silicon rubber abrasive disk for this sort of work. Don't forget the neck here either, the Adam's apple and the throat itself may become a little more prominent and the sterno-mastoid muscles that run from under the ears to the inside points of the clavicles will be much more noticeable because of fat disappearing from the areas to either side of them. Since I presume this is a 1/32 head I wouldn't worry about sculpting lines, they are usually quite slight at this age and I would generally add them at the painting stage if at all. A possible exception would be the crease from the outer edge of the nostrils down to the jowls, but you can also accentuate this with paint quite effectively.
With advanced years it is important to note that ears and nose both grow as Janne pointed out, this isn't an illusion based on other areas shrinking
As for the putty to use I use MS for this sort of thing but I'm working over itself, it can be tricky to get it to stick to resin in small quantities, in which case Kneadatite or Apoxie might be better choices.
Einion