Hi,

I'm not entirely clear on age order of the princes.
I think based on oldest to youngest it's:
1) 1st Prince - Li Chen Ru
2) 2nd Prince - Li Cheng Ze
3) Crown Prince - Li Cheng Yu
 4) 3rd Prince - Li Cheng Ping

Is this correct?

And was it explained why crown prince was chosen to be CP over his elder brothers?

Thanks

1) 1st Prince - Li Chen Ru ; his mother is related to Dongyi, he cannot be Crown Prince or else he might held too much power. 

Although I am not sure if CP is older or 2nd Prince is.
2) 2nd Prince - Li Cheng Ze ; His mother likes to read books, I don't think she was interested in court politics. Emperor uses him as a whetstone for Li Cheng Yu.


3) Crown Prince - Li Cheng Yu ; I think his mother was more favored and so he was chosen. Might also be hierarchy related, as the Emperor married the Princess (daughter) of the previous Emperor of Qing, I believe.


 4) 3rd Prince - Li Cheng Ping ; too young at the time for the crowning of Crown Prince title.

Thank you for the explanation

ETA: I am rereading the novel and few details missed on first read, now I care to pay more attention and sharing here. This explains my numerous updates here :)


It is mentioned that there is 1 year difference between CP and second prince. (SP older)
It is mentioned that FX is a bit older than 16 when he enters the capital (he was already 16 when WuZhu asked his age + 2-3 months travel to arrive at the capital)
it is mentioned that Crown Prince was 18 (when FX arrived at the capital)
Fan Xian was 16-17 when he left DanZhou for the capital
It is mentioned that third prince is 9


So the order should be:

Talented Lady Ning (Mother of the Eldest Prince) - Mother is a Dongyi's princess
1. Eldest Prince (大皇子) aka First Prince aka Noble Prince Li Chen Ru 

“Ning the scholar is from Dongyi. The Emperor brought her back from his first military expedition to the north. I’ve heard that His Majesty was wounded on the battlefield, and Ning the scholar tended to him night and day, so His Majesty freed her from her bondage as a slave and brought her to the palace, and she gave birth to the prince. But she is not from the Qing Kingdom, and although she saved the Emperor’s life, and gave birth to his eldest son, there was still no way for her to win the favor of the Empress Dowager, so naturally she could not become Empress. And she was an imperial consort, but ten years ago it seemed like something happened in the palace, and His Majesty was very angry, and stripped her of her position, so she dropped to the rank of “Talented”

***

Imperial Consort Shu
2. Second Prince: Lǐ Chéngzé (李承泽)
19 years old when Fan Xian entered the Capital at 16

***

Empress  (Mother of Lǐ Chéngqián)
3. Crown Prince  Lǐ Chéngqián (李承乾)
18 years old. The emperor mentioned 1 year difference with second prince.

“The prince is the son of the Empress. He is the third youngest among the royal siblings. He is eighteen.”

“The Crown Prince’s retinue stopped, and the palace bodyguards also stopped Fan Xian. Fan Xian parted then curtain and looked out, his face expressionless. He saw the only heir to the throne upon the bright yellow carriage—soon to be the most powerful eighteen-year-old in all the land”

***

Mistress: Yè Qīngméi (叶轻眉)

4. Son: Fàn Xián (范闲)
Born: Qing Kingdom’s 57th year (Taiping  Ye Quingmei assassination and rebirth of Fan Xian)
16 years old when Fan Xian entered the capital
17 years old Fan Xian married Wan'er
20 years old Fan Xian took the position of Director of the Overwatch Council

***

Queen Yi Quipin (Mother of Lǐ Chéngqián and Li Chengping )
5. Third Prince: Li Chengping 
8 years old  (8 years younger than Fan Xian) - "He is a certified pimp at the age of eight"
“What about the youngest prince? Is he a difficult one too?” “Wenyun is only eight.

When Fan Xian arrives at the capital city 

20  first prince
19 second prince
18 crown prince
16 fan xian
8 third prince

it's funny how coincidental it is, the emperor had 5 women, born 5 sons (each woman had 1 son), and all of them are male without girls.