Taking test at a young age

Lets say somebody wants to take test at like 19 or around that age. What is actually going to harm them by doing that. A lot of people on reddit just tell them not to because their body has to figure itself out, but nobody comes with any science backing it up. Just saying it will fuck up their brain and what not. Can somebody actually tell me what its physically doing to the body which will ruin development and so on. If u just keep estrogen balanced without it too high or low and ofc pct at the end, what could go wrong?