By the RAW, you probably can't create anything other than adult versions of most things. This is because age is usually handled via templates and you can't apply templates using polymorph spells unless said spell states otherwise.
You can make something "appear" younger or older, but it wouldn't gain any of the mechanical benefits/penalties associated with it (eg. Wizened Appearance). So the polymorphed creature would still age normally based on whatever their actual creature type is. POA and the spells it's based on don't actually change your creature type even if you have the form of said creature type.
This means that even if you turned a pebble turned into a human and found a way to make the duration permanent, it won't actually age and would never become venerable or even die of old age.
I mean, depending on your race you might be able to do something along those lines
For example, a halfling could use Twine Double + Youthful Appearance to make something that is the same size and appearance as a human toddler. Assuming of course that the only goal is to create a decoy.
There's also Major Image (which has sight, sound, smell and heat). But I assume you're looking for something that's more then just a straight up illusion.
edit: re-reading twine double, you'd have to cast Youthful Appearance on yourself and depending on your DM that may or may not work. I wanted to use Shadowself to make copies of my party by first making myself look like said party members, but the DM ruled that all copies I used would look like my normal self, regardless what spell effects I was under at the time of the casting. I also realized that my plan wouldn't of anyway since I was a gnome, meaning I would of created miniature versions of my party members....
Best. Beguiling gift. Ever.