The earlier its diagnosed, the more severe it tends to be. If someone has schizophrenia triggered under the age of 25, the massive shift in the balance of neurotransmitters has a significant effect on the continuing development of the brain. The frontal cortex (the executive function, intelligence/wisdom, and common sense part of the brain) is the last part to finish developing. That's why you can have teenagers and college-aged kids that are extremely smart academically, but absolute morons when it comes to decision making and self-restraint.

Schizophrenia is characterized by massive overloading of dopamine to the point that the brain malfunctions, and the medications used to treat it (anti-psychotics) mostly work by dulling the effects of dopamine and limiting its production. Finding the right anti-psychotic and right dose of that drug can take a lot of trial and error, and that's all time lost for ongoing development of that person's brain. Dopamine is a very important neurotransmitter, so if someone has severe schizophrenia requiring strong dopamine inhibition, they can end up with a lot of nasty side effects.

The medications have long term effects too and there's kind of a maximum amount of time you can be on an anti-psychotic before you start having a form of medication-induced Parkinsonism. If someone's schizophrenia gets triggered then diagnosed and treated earlier, it means they are going to start having those Parkinson's symptoms that much earlier.