I'm clueless about the Jedi, and I wanted to know - can you be a Jedi at fifteen? When you started training at five? If your midichlorian count is 13,200? How old do you at least have to be to be a Jedi? How long does it take to train?
If you show an aptitude for the Force, you get trained from birth.
Becoming a Jedi required the most profound commitment and astute mind, the life of a Jedi was one of sacrifice. To hinder transgression, those who showed an aptitude for the Force were taken directly from birth (or soon afterward) to train in the Jedi Temple headquarters on Coruscant or at smaller Jedi Enclaves as Padawans.
It was common for children studying to become a Jedi, to not be selected as Padawans and to be shifted to other services at the age of 13 (Jedi Initiate Trials).
Not all students were promoted to direct study as Padawan under a Jedi Master. In the days of the Old Republic, it was common practice for younglings who were not selected to be Padawans by the age of 13 to be shifted into one of the divisions of the public service organization, the Jedi Service Corps.
Becoming a Jedi Knight take a very long time, and it's not certain that one will become a knight in the end. They have to pass a trial first.
Jedi Knight – Following a period of nearly a decade of one-on-one training with a master, a disciplined Padawan was eligible to attempt the Jedi Trials. Upon successfully completing all tests put forth, a Padawan's braid was severed by the High Council and their life as a Knight began.
Sometimes exception were made, like in the case of Obi-Wan Kenobi, Anakin Skywalker and Luke Skywalker.
Exceptions were sometime made in the passing of the trials, but they were far and few between. In times of turmoil, the trials could be bypassed at the discretion of the High Council if the Padawan had shown great skill, courage, or dedication to the Order.
So, lets analyze this.
To become a Jedi you:
- have to show signs of the force
- train until a certain age as youngling
- be selected as a Padawan at the age of 13, pass as a Padawan in the Jedi Initiate Trials
- train for a very long time (close to a decade) with your master
- pass all your trails in the Trials of Knighthood (Trial of Skill, Trial of Courage, Trial of the Flesh, Trial of Spirit, and the Trial of Insight)
- become a Jedi Knight in the end
In simple math, 13 + 10 = 23.
To finally answer your question, you can't become a Jedi at the age of 15 (in normal circumstances). You may however become a Jedi at 23 or past that age. If it happens below that age it can only be in a special case.
Obi-Wan Kenobi's midichlorian count is 13,400. He didn't get selected to become a Padawan, so it doesn't determine ones success in becoming a Jedi immediately.
The training process is a very long one and it takes a lifetime to properly master the skills of the Jedi.