You call it confirmation bias but he does not look 18 or 19 at all. There is a long history of age cheating in Kenyan athletics. There is only around a 50% rate of hospital births and legal registration of birth certificates, as I showed very recently here, is still very limited. He had every incentive to identify as 17/18 because that got him the ticket to Europe and talent alone doesn't get you to 3:34. That takes years of training. Even Hobbs Kessler had been running throughout high school before his 3:34 and he was a sub-4:10 miler type the previous year, in a time trial. You can go back and look at phenom after phenom in Kenya and see their best times within two years of making it internationally because they have to be close to a peak to get out if they're track runners.
Noah Ngeny, 20 when he ran 3:28.73 and 21 years old when he ran his 1500 pr and won the Olympic gold. Out of the sport by 2001-2.
Ronald Kwemoi, still lasted at 18 when he ran his pr 3:28.81.
Daniel K. Komen, 21 when he ran his pr of 3:29.02, ran 3:29.72 at 20.
Silas Kiplagat lasted a few years but he was listed at 20 when he ran his second best time of 3:29.27 (he eventually ran 3:27.64 at 24). The IAAF lists a 1500 in Kenya of 3:39.1 at altitude at 5,000 feet for the previous year at 19 and that is it. At 27, he ran 3:32 and then never broke 3:40 again. How many people at the top are done by 28 years old?
Daniel Komen, 3:29.46, his lifetime pr, at age 21.
Augustine Choge 3:29.47, his lifetime pr, at 22.
Caleb Ndiku, 3:29.50, his lifetime pr, at age 20.
Nixon Chepseba, age 21 when he ran his lifetime pr of 3:29.77.
Need I go on? There have been some Kenyan athletes who had reasonable listed ages when they ran their top times, guys like Timothy Cheruiyot, who ran his best in his mid 20s, and Laban Rotich ran his best at 29. But there are a lot of guys like Reynold Cheruiyot who burst on the scene at almost unbelievable ages with no track record of performances leading up to that point and indeed many are older than they say. So, we can't just predict a host of major improvements for them. As seen above, all of the Kenyan 1500m prodigies under 3:30 fizzled out early.