How accurate are diagnoses on Autism to Children under 3 years old? I have a son with autism. He was diagnosed under 3 years old. 2 years old to be exactly. After I doubted several pediatricians that claimed that wouldn't be a valid diagnose after 5 years old. I discovered by myself that it wasn't entirely true. I've read that specialists would diagnose under 2, or even, babies. I want to know because other parents say their doctors say to wait from 2 to 3 years old or even more.
There was a study which examines the exact question you posed.  This study, which was conducted by Stone et al. (1999) compared accuracy of diagnosis of autism and a pervasive developmental disorder not otherwise specified (PDD-NOS).
With regard to autism, the study suggested 2 conclusions:

(1) the identification of an autism spectrum disorder can be made reliably in children below the age of 3; and (2) the specific diagnosis of autism can be made reliably in children under 3 years, as long as the diagnosticians have experience evaluating young children.  The latter conclusion is presented more tentatively, given the limited sample of clinicians employed in the present study. Further study of the impact of experience on diagnostic reliability at young ages, using lager samples of clinicians, would shed additional light on the issue.

Regardless of the initial diagnosis, the large majority of children remained on the autism spectrum at the 1 year follow-up.

Of the children who received a diagnosis of autism at age 2, 96% remained on the autism spectrum at age 3, and 72% retained the specific diagnosis of autism. Although the results suggest that symptomatology improved for 28% of these children, only 1 child (4%) improved to the extent he &quot;left&quot; the autism spectrum.

They also pointed out that diagnostic stability did not vary as a function of whether the same or a different clinician made the Time 2 diagnosis, proving further support for the robustness of the diagnosis.
Additional Information after issues surrounding debate were raised.
With regard to the debates over autism, there is a factsheet on the debates at autism-help.org and the debate covers the causes of autism (genetics etc), lack of definitive test for autism

due to the significant variance of symptoms among people with autistic traits, as well as a lack of knowledge about the etiology of the condition. Those with Asperger syndrome may be highly functional cognitively but lack ‘social’ skills, whereas others with so-called “profound Autism” may be non-verbal and deficient in elementary skills.

and there are issues surrounding the diagnosis of Asperger’s syndrome as opposed to autism.

[M]any autism spectrum adults and neurotypical (non-autistic) allies in the autism rights movement do not consider Autism to be a disorder, but simply a different way of perceiving and behaving. These people believe that at least some of the difficulties encountered by people with Autism may result from prejudice and lack of accommodation from society.

Stone et al. (1999) is cited in over 200 papers including many over the past 5 years.  One of them is Roberts, et al. (2019), where they also point out that ASD can be reliably diagnosed by 24 months, although the average age of diagnosis is 2 years later.
Clark, et al. (2018) also highlights that early diagnosis of Autism Spectrum Disorder is considered best practice, increasing access to early intervention.  They investigated the school age outcomes of children who received an early, as opposed to later, diagnosis of ASD.  The abstract highlights that:

Children diagnosed early accessed more intervention, demonstrated better verbal and overall cognition at school age, were more likely to attend mainstream school and required less ongoing support than children diagnosed later.

So while raising the debates about autism diagnosis in the last 20 years is a valid argument, it does seem that the 1999 paper still stands as valid, even though changes were made in the DSM with the publication of DSM-5.  But then again, when you look at following studies,

DSM-5 resulted in 47.79% fewer toddlers being diagnosed with ASD compared to those on the DSM-IV. Toddlers diagnosed according to DSM-5 exhibited greater levels of autism symptomatology than those diagnosed with DSM-IV, but the latter group still exhibited significant levels of autism symptomatology(Matson, et al. 2012).

So has the change created a problem where some are no longer diagnosed when they should have been?  That would be open to opinion I believe.
References
Clark, M. L. E., Vinen, Z., Barbaro, J., &amp; Dissanayake, C. (2018). School age outcomes of children diagnosed early and later with autism spectrum disorder. Journal of autism and developmental disorders, 48(1), 92-102. doi: 10.1007/s10803-017-3279-x
Matson, J. L., Kozlowski, A. M., Hattier, M. A., Horovitz, M., &amp; Sipes, M. (2012). DSM-IV vs DSM-5 diagnostic criteria for toddlers with autism. Developmental Neurorehabilitation, 15(3), 185-190. doi: 10.3109/17518423.2012.672341
Roberts, M. Y., Stern, Y., Hampton, L. H., Grauzer, J. M., Miller, A., Levin, A., ... &amp; Estabrook, R. (2019). Beyond pass‐fail: Examining the potential utility of two thresholds in the autism screening process. Autism Research, 12(1), 112-122. doi: 10.1002/aur.2045
Stone, W. L., Lee, E. B., Ashford, L., Brissie, J., Hepburn, S. L., Coonrod, E. E., &amp; Weiss, B. H. (1999). Can autism be diagnosed accurately in children under 3 years?. The Journal of Child Psychology and Psychiatry and Allied Disciplines, 40(2), 219-226. doi: 10.1111/1469-7610.00435