Do you mean that 2% is not a clinically significant figure? Could you explain why? (I'm not familiar with clinical significance)
In the book The Emperor's New Drugs, about the efficacy of anti-depressants (or lack thereof), Kirsch gives the example that if you had a group of 500,000 people and found that smiling frequently increases life expectancy by 10 seconds on average, this may well be statistically significant because the study population is so large. However, it's not clinically significant because no one really cares if they live an extra 10 seconds, certainly not enough to change some major aspect of their behavior at least.
In this case, if the chances of having kids with autism or schizophrenia increase by/to? at most 2%, as the article states, then in practice this shouldn't really deter anyone from having kids. Thus, while the findings are statistically significant, they are not clinically significant.
It's determined entirely by perceived significance. For each type of drug, there is usually some agreed on metric within the field that determines what constitutes clinical significance. E.g. a blood pressure drug might have to lower blood pressure by 5% or 10% or whatever. And often times the metric isn't even relevant, as in the case of a reduction in blood pressure vs. a reduction in total mortality / morbidity.
However, at the end of the day what the academic community considers clinically significant doesn't really matter, it's your own preferences and values that should be your guide. Especially since in practice everyone is basically out to trick you in order to make more money, from pharma companies to newspapers to academic journals to professional organizations (AMA, APA, etc.), etc. That's why it's important to really understand this stuff forwards and backwards and to keep your wits about you.
Keep in mind that the overall risk of autism is 1 in 88, or 1.1%, and the overall risk of schizophrenia is about 1%. So it's not like you get a free pass if you have kids earlier. Also, there are other benefits to having a child later in life in terms of things that benefit the child, even if there are more neurodevelopment risks and disadvantages at the biological level.