Use liquify tool to enlarge the head the pucker tool to smaller eyes and bloat for the mouth, I study a little anatomy of a child and a growup to understand the changes.
The adult has a more big chin and the eyes and mouth on time expanded.
step 3 of 10
to add texture to the face the most important step I think, duplicate the layer from step 1 after liquify then
image/apply image on blue channel on multiply mode, the blue channel usually is the most noises channel (hate this channel :) then go filtel/other/hight pass and apply something like 6-10 pixel then change the normal mode to overlay on this layer.
step 4 of 10
the color on adult is much more desaturated so I apply a selective color on this settings;
black
black +7
neutral
black +7
cyan +17
whites
black +40
cyan -14
step 5 of 10
the hair on adult is darker so apply curves then mask the face .
step 6 of 10
hue/saturation on this settings:
reds
saturation -30
yellow
saturation -48
greens
saturation -90
step 7 of 10
make a new layer empty go edit /fill 50% gray and start to sketch the lines the wrinkle etc with burn tool on 10 %
step 8 of 10
in this step a remove with clone tool the hair below the eyebrow and I used again the liquify tool on the face :)
I know I work very messy:))
step 9 of 10
curves again
step 10 of 10
again new layer fill with 50 gray this time I used and dodge tool