When would it be ideal for me to begin medically managing my child?
Medical management of autism can be at done at anytime, since it works to alleviate the symptoms associated with ASD. This, however, should be strictly done under a registered medical practitioners supervision.
Different countries have different standards regarding the prescription of some drugs such as antidepressants or SSRIs, so depending on the country's laws, a licensed pediatrician or doctor can prescribe the medication that is to be taken.

Other Questions
How can medicines help my child with Autism?
Which medicines would help my child with Autism?
Are there any oral supplements that would help my child?
Is there anything more I can do for my child at home, in terms of medications?