Well, again, to me these characters already look like they could pull that age off, so it's hard for me to give specifics on what to change about them. You're certainly going to want to use a program like GIMP, Photoshop, or Sai and make your changes by hand with a brush tool.
I can't speak for GIMP, as I've never used it, but between Photoshop and Sai, for this I would say Photoshop is better. If only because Photoshop's brush tool has a naturally more aliased line that I find matches the art on the face pictures a bit better. ((Even though I prefer Sai overall between the two - its lines are just TOO clean for this))
Make yourself a new layer to draw any changes you make on, so you can easily see the original and your edits by making that layer invisible. Make your changes. You'll want to use the eyedrop tool extensively and I use a 1px brush (if you are working on the 96x96 face set image and not the fullsized bust art I mean)
The eyedrop tool will ensure you keep with using the same colours as the original image. Also, but drawing on a new layer you can easily erase changes you decide you don't like, or eyedrop colours that you've covered over.
Here is an example. I did not try to make her look older (as again, to me she always easily passes as 20 as far as anime art goes) but I tried to give her a face that suggested a more mature character compared to her original expression:
Original:
Edit:
What I drew to change it:
So again, the third image is what was on my second layer and what I drew to turn the 1st image into the 2nd.