Many fans have asked over the years why Goten and Trunks take so long to grow in dragon ball. Sure, we all know about Trunk’s future time with the androids and Goku Black.
However, in the original timeline of Dragon Ball, both Goten and Trunks are still kids. They have not yet hit puberty and fans want to know why? Well, there is a few reasons why we have not seen them grow up in the anime and this is one of the reasons why.
The Reason Goten and Trunks Take So Long To Grow
Both Goten and Trunks have grown in the past, however, during Dragon Ball Super series they were actually 13 and 14 years old. However, the Saiyan race physiology makes them stay younger than how humans would.
It is also the reason why son Goku looked so young for the most part of the original Dragon Ball. No one knows if they will ever age up to 18 – 20-year-olds based on Toriyama’s stories telling in the near future.
By growing Goten and Trunks up too fast, they would have to slow down the pace of the story to get them up to a level where their father’s strengths are.

Why Goten And Trunks Have Not Grown Up
The reason why Goten and Trunks have not grown up yet is that their current designs are cute and most importantly, marketable.
Toriyama and the Dragon Ball Super Production team would not dare to mess with anything to disrupt their core appeal and merchandise. It does not matter what.
In fact, one of Dragon Ball’s biggest strengths as a story was its willingness to change the formula, characters, and reinvent itself, to have multiple characters grow and evolve over the course of the franchise.
Dragon Ball Super is pretty much terrified of major risk. That is why DBS is sectioned off in this risk-free safe zone in between the defeat of Majin Buu and the manga’s finale.
They prefer to keep all the characters in Dragon Ball their most marketable state. Why? Well, if they allow Goten and Trunks to hit puberty would be a risk. Also, allowing other main characters like Bulma to become old or middle-aged would be a risk as well.
allowing main characters like Krillin to keep his hair grown out would be a risk. Taking Goku out of the orange and blue clothing he has been for years would be a risk too. For Dragon Ball, everything has got to be safe and original.
So to improve or change anything else would be to risk alienating the audience apart from new transformations.
S, there you have it this is one of the reasons why Goten and Trunks have not grown yet in the anime and probably they never will.