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Smoothing out wrinkles in a photo is pretty short work in Photoshop thanks to the healing brush tool. Here is the photo I will start with.
Open the healing brush tool (shortcut key J). I will focus around the eyes first. You will need to find a spot next to the area that you are planning on "healing". I like to keep this selection close as it will more closely follow the lighting of the area which will keep it looking natural. Find a spot and Alt click the area.
Now you just need to "paint" along the wrinkles under the eye. I like to follow the curve of the wrinkle with the brush to maintain a natural look. We don't want this to look like it was Photoshopped. Notice the little cross hairs next to the brush while you are painting. This is the area that the brush is copying.
When you let off the mouse it will blend in with the rest of it's surrounding. This looks a bit better.
All you have to do is repeat this procedure wherever there is a wrinkle. Don't forget the cheeks and neck. Here is what I ended up with.
OK, that is not too shabby. I think I will take it one step further and use the blur tool just a bit. Open the blur tool (shortcut key R) and set the stroke strength to 30%
Paint along the lines that were left by the healing brush. Make sure to follow the natural curve of the face and lines. This will soften up any remaining wrinkles.
Here is a before and after.