She would not probably disappear, as those creams are marketing bullshit.

Yes. Obviously.

Only her skin will disappear, she'll be only meat and bones.

No, silly. Just her skin. However, at that age, she is constantly producing new skin at a faster rate than the aging cream is reducing it, so it would hardly be noticed. I could take one for the science team and study her skin intently if you would provide me with a 19 yo girl specimen and the cream.

Oh, I got a spam message in the mail the other day about just that, about a cream that would take 20 years off... I don't want to be a teenager again.

Only if she's only skin seep.

Yes, and now I believe the phrase "I'm gonna wipe that smile off your face" can be a real threat.

Isn't that the plot behind the 1940's movie "The Invisible Woman"?
Spoiler alert!
"At the end of the film it is revealed she has married and become a mother. To top it off, she and the professor learn that her treatment has apparently become hereditary—as her infant son vanishes upon being rubbed with an alcohol-based lotion."

If you're only 19 and use that.. you deserve to disappear :P better to use it at age 40 and look 20 again xD

If a 19 year old girl uses cream clinically proven to reduce the age of your skin by 20 years, does she disappear?

The cream? Yes, of course. Although technically it should be "it" not "she"

Possible outcomes:

1) her skin disappears and she dies either from body failure or from suicide because having skin is too important in today's society
2) we have a first hand look at the reverse process of matter creation
3) her skin gets a bit healthier because ads were distorting the truth and the skin renewing effect is actually proportional to the person's age

I vote for 1 because it's the most realistic result.

This might result in a bug.

View attached image.

Thanks