Sageretia - Determine the age?
 

I am an absolute beginner in the art of bonsai (started 3 days ago, in
fact) I've purchased a Sageretia from a local gardening store. After
reading some threads/sites I realize that my bonsai might be on of
those 'mall bonsai' that are very easy to die, though I still hope to
keep its company through this winter. The store owner couldn't give
much information except to submerge it in water twice a week (bah).
There're some issues I'd like to ask for opinions & advice :)

- Age ?: The tree hight is roughly 12 inches from the base of the
trunk, which is about an inch in diametre. It's been truncated at the
top and the wound looks quite old. The trunk is not yet flaky.

- Yellowing Leaves: Eveyday some 5-10 leaves turn a bit yellowish with
small dark spots. I plucked some of them out at first, but wondered if
it has some sort of disease. Since the foliage is not thick to begin
with, I start to worry.

- Temperature & Lighting: My apartment faces North and thus gets no
direct sunligh at all. However, natural light in the room is still
quite bright for 8 hours a day. Do I need an artificial grow light
lamp? I try to place it as near the window as possible (usually closed,
using heater from time to time). My room is kept at 15-20 C. Is the
range suitable for Sageretia?

It's been only 3 days and I grow attached to this little tree already!
Please give me some advice to keep it alive and healthy :)

Leaves: http://static.flickr.com/31/62787137_5160cf8669.jpg
Bonsai: http://static.flickr.com/32/62787138_34b53c7ca1.jpg