for all ages is quite easy, really.
And since I'm lazy and tired right now, I'll just quote myself...
For making toddlers to elders behave as other ages, you need:
the Insim machine and the insim Summoner, the OMSP from MTS2, an invisible version of the OFB magisplay tray thingy, the Moveobjects on and Boolprop snapobjectstogrid false cheat codes + a lot of patience, since trial and failing is a big part of a photoshoot. (you might also need an older/younger version of the sim you're going to use)
You must have a sim in the age you want the sim to behave like (he must be saved as this age, meaning that you can't age him to this before the next step - unless you save, quit and reload the lot), and then you age him down/up (blue part of the insim machine) to the age he's supposed to look like. Then you delete him, and summon him back again with the summoner (restore family). Remember that the poses will act a bit weird for all ages, but they can now use poses for other ages - children will be able to use adult/teen poses. BUT: toddlers/children will hover in the air as older ages, while teens/adults will crouch a little as younger ages.
The OMSPs and the OFB tray is used for placing the sims higher than the poses normally are.
Remember that you do this on your own risk, meaning that saving the game after messing around with especially insim and the magisplay tray MIGHT do strange things to your game (I lost a really cute toddler because of the tray - luckily I didn't save, so I got him back). I try to avoid live mode when playing around (That's how I lost the toddler), and take everyone off the trays before changing to live mode again.
I recommend trying the techniques on a test lot the first times you play around with them.
But for doing it with babies, you'll need some knowledge of SimPE. I think there's a tutorial somewhere, possibly in the tutorial section here at Insim.
Babies are more like objects than sims, and the game does not really start to recognize them as sims until they reach toddler stage. You can't use the method above for the babies. If you accidentally do it, you will end up with toddlers (or other ages) behaving as babies, and this will result in error warnings, and faulty simmies that can not be interacted with. IF you use this method, remember to ALWAYS save the game AFTER aging up a toddler from baby stage. If you delete the toddler, and summon him again, You will get the error warnings.
I think there should be a tutorial somewhere about how to do it, but remember that this CAN mess up your game.