I think the whole thing with when to use the preterite/imperfect is totally confusing.
to say "when i was 12 years old," would you use the preterite or the imperfect (cuando tuve/tenia 12 anos)? and why?
could somebody please provide some explanation and examples of when to use either verb form?

thanks so much this would help me a LOT.

use the imperfecto when the action was "taking place" in the past - Estaba estudiando cuando ladró el perro - I was studying when the dog barked.

Use the preterito when the action "ended" in the past - Estuve estudiando ayer. - I studied yesterday.

to say "when i was 12 years old," you use imperfecto since you are telling something that "continually happened" in the past:

Cuando tenía 12 años me divertí mucho jugar con mi osito. - When I was 12 years old I enjoyed playing with my teddy bear.

The preterite describes a completed action event in the past. Like "He bought a book" The imperfect tense is used for what somebody or something was like (including age), what used to happen (we watched TV every night) and what was happening/what somebody was doing (it was raining when I came home).

Preterite:
a single action
something completed/done
something done once
the beginning/end of an event
an action that interrupts another action

Imperfect:
a repeated action, happened more than once
an unfinished event, still going on
descriptions - people, places, things
to tell time - hour, day, month, year

i always have the same problem... but these link would help you a lot..

http://faculty-staff.ou.edu/L/A-Robert.R.Lauer-1/PretImp.html

Oooh! Loaded, complicated question!

It depends on the context.

If you are saying, "when I was 12 years old, I went to church every Sunday", (as opposed to when you were 11, you weren't required to, and when you were 13, you went on, say Saturday,) it's tenía, because you are talking about a long period of time. Actually, it's almost always tenía, becaause it's tener, and tener is almost always used in the imperfect...and because most of the time, when you are talking about your age, you are using the imperfect.

The only times you would use tuve with age, would be if you were referring to that year in terms of a longer period of time. That would be rare. I just can't even come up with a good example.

And, I hear you. Pret./Imperf., was when I really had to start working for my grades. It was one of the hardest concepts to get, and even now, years and YEARS later, with a good grasp of the rules, I sometime have to stop and think and correct myself...It's just a tough one for us English speakers.