Why did Dragon Ball Z's present-day Trunks reach Super Saiyan before his future self? Following the destruction of Namek, a teenage Trunks arrived from 20 years in the future to warn Goku of the apocalyptic Android threat that would emerge in 3 years' time. Having already revealed himself as a Super Saiyan (the first of many to come after Goku) during a one-sided fight against Frieza, Trunks challenged Goku to a sparring match, only to find out that the finger can be just as mighty as the sword. Trunks explained he was the sole survivor of the future Z fighters, most of whom perished at the hands of the Red Ribbon Army's deadly Android duo. Much to Goku's amusement, the mysterious stranger also announced he was the offspring of Vegeta and Bulma, due to be born in 2 years.
Although some of Future Trunks' predictions changed unexpectedly, his conception remained unaffected, and a baby Trunks is first seen at the start of the Android Saga. As the purple-haired infant grows, however, it becomes clear modern Trunks' personality and abilities are very different to those of Future Trunks. For instance, present-day Trunks mirrors his father's arrogance and pride, whereas his future self is more cautious and humble. Despite this, the most significant difference between the two versions is when they both achieve their Super Saiyan status. In the Dragon Ball Z special "The History of Trunks," the teenager only acquired his blond-haired power-up after the death of his mentor and best friend, Gohan, killed by Androids 17 and 18. However, the much younger present-timeline Trunks casually transforms while training with Vegeta at only 8 years old, noting that he and Goten had already obtained this power a while back. Even Vegeta remarked on how young Super Saiyans were getting nowadays.
This poses a notable inconsistency in Trunks' already topsy-turvy timeline. The issue isn't how easily Trunks and Goten followed in their fathers' footsteps by becoming Super Saiyans, but how hard Future Trunks had to train for the transformation compared to Kid Trunks. Similar to Goku's first transformation after losing Krillin, it took Gohan's death to trigger Future Trunks and allow him to break through the Saiyan barrier, whereas his younger self managed to achieve the transformation without the same rigorous training or the high-pressure environment of the Android apocalypse.
A few ideas have been proposed to account for Kid Trunks' early power-up in Dragon Ball Z. One theory posits that since present-day Vegeta was a Super Saiyan when Trunks was conceived, he essentially passed on the power to his son (this theory has also been applied to Goku and Goten). On the other hand, Future Vegeta might not have attained Super Saiyan without the intense training he undertook ahead of the Androids arriving. The most common explanation suggests that because Future Trunks didn't have a father to train with or Goten to grow up alongside and compete with, this halted his development, despite the best efforts of Gohan in the future. Finally, the plot hole could simply be another unintended consequence of fiddling with the timeline in the first place.
While the canon explanation remains up for debate, the real-world reason Kid Trunks reaches Super Saiyan before Future Trunks is more clear-cut. In Akira Toriyama's original manga, Future Trunks' was already a Super Saiyan before Gohan died, with the exact time he achieved the form not stated. For all readers know, Future Trunks turned Super Saiyan aged 8, just like his alternate self. When the "History of Trunks" story was adapted for TV, the anime altered Trunks' ascension to coincide with Gohan's death, most likely for dramatic effect. This change certainly made the episode more poignant, but ended up backfiring when young Trunks is revealed to already be a Super Saiyan. Such are the perils of changing the Dragon Ball Z source material.