What happens is this:
When you give an adult pet the extract, it gets the effect Ageless Adolescent. If you apply this effect, it will have the picture of the adolescent and the age of the adult. You still have to feed it, and it will still grow in age, but it's picture will remain the same.
If you give it two potions, you get the effects Ageless Adolescent and Ageless Baby. You will be able to apply either of the effects. You can then have your adult creatu have the picture of a baby, or an adolescent :D