Why does Red Boy never grow up?

The Red Boy in "Journey to the West" is the son of the Bull Demon King and the Iron Fan Princess. Although he is only a seven or eight-year-old child, his magic power is not small. Even Sun Wukong asked Guanyin Bodhisattva to Only with help can Red Boy be restrained.

Actually, although Honghaier looks like a seven or eight-year-old child, his actual age is more than three hundred years old, but why does he always look like a child? There are different opinions.

One theory is that when Princess Iron Fan was pregnant, she asked Tathagata for a promise that their son would always be eighteen years old. But one time, Sun Wukong came to visit the Bull Demon King and told a joke with the Bull Demon King, which made Princess Iron Fan laugh so much that Princess Iron Fan was born prematurely. Before the end of the month, the red baby was born, causing the red baby to not grow up.

It is also said that when Princess Iron Fan gave birth, she gave birth to a stillborn fetus. The Bull Demon King loved her son very much and was unwilling to give in. He asked the Supreme Lord to help him save the baby, and the Supreme Lord used the elixir to save it. He got Red Boy, but told the Bull Demon King that Red Boy could only grow up to seven or eight years old, so he stopped growing. He gave him an immortal body and the ability of Samadhi True Fire.

There are many opinions, but this may be the need of the plot of "Journey to the West". If the red boy grows up, he will not be accepted as a good boy by Guanyin Bodhisattva.