What is the way to make someone look young or old, fat or skinny on Photoshop?

Practice.
I don’t use Photoshop, but I know that it has a tool called “Liquify” that is popular for making people look fatter or thinner. As for aging or creating the illusion of youth, that would require some creative finesse, I would think. Smoothing wrinkles or drawing them on. The same goes for age spots or dark shadows, loose skin, etc.

Thanks!

Disclaimer: I don’t have much experience with photoshopping, but a fair amount of experience with drawing people.

Age is mostly a matter of proportions, at least until around 20 years. Younger children have larger eyes and a smaller nose and mouth in proportion to their head, a larger head in proportion to the rest of their body, shorter limbs, and a few other things. Methinks it would be a real challenge to photoshop an adult into a child, even for an experienced shopper.
Among adults, the differences between a young adult, a middle-aged person and an elderly person are less pronounced, meaning it’s easier not to mess up, but also more difficult to make the difference clearly visible. But young adults are generally slimmer by comparison, and older adults have wrinkles¹ and greying hair and the like. Try working with that sort of thing.

Extreme differences in bodyfat percentage are visible in specific places that are also tricky to get right. You’d need a reference.
I can’t really find any at the moment, but try looking for anatomic drawing tutorials.

Incidentally, this looks relevant.²
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¹ You’d have to study some old people’s faces to learn exactly where the wrinkles go, so it would still take some skill. Shopping a wrinkled face into a smooth one is easier.
² It also looks rather dreadful, IMHO, but you be the judge.

Photoshop is a powerful program. Too many tools to mention here and do it justice.
Some good ones are the blur tool for wrinkles, eraser tool to get rid of jowls and stuff under the chin, stamp and repair tools for blemishes and brown spots, etc etc and etc.

Photoshop takes time to get comfortable with. I’ve been shop’n for years and still find myself looking over tutorials again to find that 1 tool or something I missed that’s not giving me the effect I wanted. There’s some cool tutorials Here if you decide to just play around with ideas. It’ll help you get to know your tools a little better.

You haven’t found those thinner/fatter and younger/older buttons yet?!? Geez!

You’ll need about 10 years of practice to actually do it “convincingly”

Also, by coincidence I happen to be checking out the site and if you’re not familiar with worth1000.com They have contests in numerous categories also forums, so it would be an excellent place to check out. The main reason for this quip though is because recently they had a contest on doing this exact effect you’re talking about.

Here’s a quick copy and paste* of the rules.

As children (and especially teenagers trying to get into bars) we all wish we looked older. As adults we wish were children. Looks like image-editing is the closest to a fountain of youth we will ever come. In this contest you will take anyone or anything famous, dip them in the magical waters of Aviary, Photoshop, Pixelmator or any other program and pull out a different aged version of themselves as in our themepost.

The rules of this game are thus: You may use any famous person, but must age them (either younger or older so that a difference can be seen easily). You should include the name of the person in your title.

Please, DO NOT post WIPs in your comments. Post them in the forums after the contest has ended.

Have fun, be creative!, keep it clean, follow the Arena Rules please, and be careful to avoid using cliches. You will have 48 hours to submit so make your entry count! As always, quality is a must. We’ll remove poor images no matter how much we like you.
* via worth1000

Here’s the entrys
Also if you click the link under an entry you get to see what they used to create the photo if they have posted the sources.

Enjoy and have fun. :)