I just...did

There's not a lot more to it.
I start the guy out as the http://img28.imageshack.us/img28/5382/screenshot270s.png actor at a scale of 0.3. With http://www.creationkit.com/RegisterForSingleUpdateGameTime_-_Form, every seven game-days I "grow" him by 0.1 using http://www.creationkit.com/SetScale_-_ObjectReference. Simultaneously, on the third week I swap him with the teen actor and then four weeks after that I swap him again for the advlt actor (both are placed in a holding cell until needed). This is all handled by a http://www.creationkit.com/Quest and an http://www.creationkit.com/Alias. When he's "fully grown" he's at scale 1.2 (I was making my own direwolf companion

)
I used 3ds Max to make the cub and teen models by scaling and tweaking the vanilla advlt mesh (Flattening and widening the head, shrinking the body a bit and scaling up the paws)
Like I said, not exactly the easiest "first project", there's quite a bit of Papyrus scripting involved, but please by all means don't let me talk you out of it, I just recommend the official tutorial series first so the ck doesn't look quite so daunting
Good luck mate, welcome to Skyrim modding! I hope you enjoyed actually
playing the game while it lasted, the CK can be an addictive beast

(Edited for clarity and stupid iPhone induced errors)
- Hypno