I don't have the exact timing of Gojo's death in the manga at hand. But usually, you can find such crucial plot points by closely following the story's progression or referring to fan communities and discussions dedicated to the manga.
Li Chengqian rebelled in the 17th year of Zhenguan (AD 643), when he was already an adult. However, his exact age was not mentioned, so it was impossible to determine his age at that time.
There was no clear record of how old Li Chengqian was when he became a cripple. The history books only mentioned that Li Chengqian suffered from leg problems at the age of twelve and fourteen, but did not specify what caused it. Later generations had several speculations about his lame leg. One thought it was hereditary because Li Chengqian's father, Emperor Taizong of the Tang Dynasty, and Empress Zhangsun also had leg problems. The other thought it was caused by his riding injury when he was young. However, these were just guesses. There was no conclusive evidence to prove the specific cause of Li Chengqian's leg. Therefore, there was no definite answer to the question of when Li Chengqian was crippled.