Got into a debate over this spell; in order to disguise herself, our Sorcerer cast this spell to make her appear to be a nine year-old girl (which, according to a website we checked, would put her about the same height as an adult dwarf, so approximately Medium-size).
It was argued that the sentence "The target cannot appear so much younger that it changes size." doesn't necessarily refer to the character's Size, but means no change in height is allowed (so about 15-16 years old is the minimum age for the Sorceress).
Size is defined in the rules, so I think the spell should be interpreted that way, as opposed to a "common English" reading of the spell, but I thought I'd see what other people think.
Look at the PRD, Ultimate Campaign, Young Characters. For human, young starts at 8 years old. Size isn't one of the things that gets alters by making a young character, so an 8 year old is medium. That would make the height of an 8 year old tall enough for medium.
http://paizo.com/pathfinderRPG/prd/ultimateCampaign/campaignSystems/youngCh aracters.html
Sorry, I guess I didn't clarify the question well enough. Does the spell not allow you to change size categories, or does it not allow you to alter your size at all (ie, going from 5' 6" to 5' 5").
#1 the spell will change height/weight based on the age.
#2 the change from #1 can never be enough to change your size.
#3 to find out how young the cut-off is for #2, you look at young characters. For humans, it's 8 years old.
So from your first example, the Sorcerer CAN cast this spell to make her appear to be a nine year-old girl, taking on the height you found for that age. This does NOT alter her size: she stays medium. She could in fact go another year younger and appear as an 8 year old.
Ok that's pretty much what I thought, some of the other players were reading way too literally. Thanks!
No problem, happy to help. :)
The game tends to use terms with multiple meanings way too often, so it's understandable. A safe bet in these cases is to assume they mean the game term and not the general usage of the word. For instance , look at disguise self. It tells you "You can seem 1 foot shorter or taller, thin, fat, or in between." Something like that would be needed to Youthful Appearance to affect directly limit height. Size should just mean size (small/medium/large).
If you have any more questions, feel free to ask.