Xia Ya's famous saying in Gundam series "Although I don't want to admit it, it's all because of the mistakes made by young people" refers to the mistakes and decisions he made when he was young, which led to a series of difficulties and challenges he faced in the following story.

In the story of Gundam, Xia Ya was a radical idealist when he was young. In order to realize his ideal, he did not hesitate to take extreme measures, including betrayal and assassination. These actions damaged his reputation and cost him many friends and relatives. With the growth of age and experience, Xia Ya gradually realized his mistakes, began to reflect on his past and tried to make up for his mistakes.

Therefore, the sentence "It's all because of the mistakes made by young people" not only expresses Xia Ya's regret and reflection on his mistakes and decisions when he was young, but also represents his understanding and reflection on life, reminding people to cherish every opportunity and make the right decision so as not to regret it in the future.