How early is too early to be diagnosed

When is the earliest children can be diagnosed? My son is 12 months adjusted, 14.5 months actual. He scored a 10 on his MCHAT. Mostly because he uses zero gestures/mimicking/language. His nicu follow up team insisted this was pretty much a diagnosis and they just want an evaluation to be certain. His Bayley developmental assessment was average for his adjusted age. Slightly Below average for expressive speech. We have an autism evaluation schedule for 14 months adjusted. I see a lot of people talking about a diagnosis from 2-5. I’m just really confused and worried.