Having a medical diagnosis for Autism isn't typically necessary before the age of 5. For your child to receive Occupational Therapy, Speech Therapy, Behavioral Therapy and in some cases Physical Therapy there is no need for a diagnosis IF YOU HAVE MEDICAID. Nor is there a need for one for your child to attend an Early Intervention School (ages 3-5 attend). What will happen in these cases is that the Early Intervention program will set up an appointment to evaluate your child. After the evaluation they will be able to tell you if based on your child's behavior you qualify for their services. They can also provide OT, PT and SLP as well as BT if your private insurance will not pay for it.
However, when your child enters Elementary School you ABSOLUTELY NEED the medical diagnosis. The reason being that the school district will not and does not have to provide the services your child needs. Say your child needs to be in a Special Ed class or in our case, the school my son will be attending has 3 classes dedicated solely to Autism students. Which is great because my child wont get lumped up with other children that just have basic developmental delays. And the teachers are specifically trained to work with Autism. If the child does well in his Autism class then the school will talk about mainstreaming them into the regular class. Your school district can also provide OT, PT and SLP. However, they are BOUND BY LAW to provide the services necessary to your child (this includes paying for your child to attend a private school if the public school cannot meet the needs of your child) ONLY IF you have a medical diagnosis.
This is also helpful if you choose to Home School your child. Say the only subject you are not confident in teaching your child is Math. You can make arrangements with the school district that your child will attend their school only for math. You can also make arrangements if you Home School to have someone come to your home for OT, PT and SLP. BUT only if you have the official diagnosis.