You can't prove a negative, that's not how experiments and hypothesis works.
If you're making a claim that early exposure causes harm, you're the one that has to prove it via the following steps:
- Set up criterias for assessment
- Decide on a test methodology
- Collect data
- Analyze results and draw conclusions
- Publish for peer review to make sure your methodology was appropriate
Congratulations, you have just been taught how to do science