Ds1 was diagnosed at 3.5yrs but I'd started looking into autism when he was 2.
At 2 he was very passive and had no interest in people. If you left the room he wouldn't notice. He had no useful language skills and had never really babbled. He didn't understand instructions or respond to his name. He had somehow taught himself to read (hyperlexia) and would type in his own name and password to use the computer. He was happiest when left alone.
At that age I didn't have a clue how to teach him anything. I couldn't see how you could teach a child who didn't understand anything. The best help came from the SALT he saw when he was 3yrs old and from the portage worker the SALT referred us to.
Ds1 is now 11yrs old and is almost unrecognisable to how he was at 2. He's in a mainstream secondary school where he's doing very well in all subjects except for his much-hated PE. :o He was discharged from SALT after his assessment at 6yrs showed that he was actually ahead of a typical child of his age. He has problems with his fine motor skills, which means he finds writing difficult, but types his schoolwork where possible. He has poor co-ordination and gross motor skills, which is mainly why he dislikes PE so much!
If someone had told me back then that one day ds1 would be as he is now, I would never have believed them.
Ds2 had a diagnosis of autism when he was 2yrs old. At that age he was known as "Hyper-Boy" because he seemed incapable of staying still for 2 seconds. He was always running, climbing and jumping. He hated wearing clothes and spent most of his time wearing just a nappy. He had some language skills, but they were mainly names of shapes, colours and numbers. He didn't really understand instructions and spent a lot of time with his hands over his ears or wearing ear defenders because he hated noise.
He was either very happy or in a raging meltdown, and could switch between those in seconds.
I used a lot of the tips that we'd picked up from ds1's SALT sessions, eg keeping language very simple. Ds2 also had a portage worker, who brought various toys and books to use with him.
Ds2 is now 9yrs old. His spoken language is quirky but very good. He likes to use long words wherever possible. He still hates wearing clothes and still doesn't like loud noise. He is no longer hyper and is happiest when curled up under a table reading a book. He's doing much better than I thought he would, but there's still a long way to go yet.