Well for starter you should pick a photo that's not Photoshopped to begin with.
I don't want to give away ALL of my secrets
but when aging a person there are a few absolutely essential things to remember.
1. Subtlety is the key. When you start out, try aging 10 years or so with small changes and progress from there.
2. You have to understand what happens physically when you age, and it's a lot more than just wrinkles.
As we age, the skin starts to lose moisture and suppleness which causes it to not only wrinkle, but droop and dull.
We start losing fatty deposits in our face, which causes our lips to thin out and our cheeks and eyes to sink in a bit.
VERY VERY important is our nose and ears continue to grow throughout our lifetimes so the older you want a person to
look, the larger you make their ears and nose.
3. Hair thins and greys and eyes and teeth turn yellow. Eyelids droop and gums recede making the teeth look larger and
leaving more space between them.
I am by no means a master at aging people, but I worked as a nursing assistant for for over ten years and had lots of time
to look at what makes old people look old. Focus on those things and your agings will start looking more realistic.
Best,
Chris