My DS was Diagnosed at age 5, and for ages, I insisted there were no early signs (he was my first, so everything he did was 'normal' to me, IYSWIM). As far as I was concerned, he was a lovely, if colicky, baby (he would only sleep with my hand pressing down on his chest, for instance, and then only after HOURS of being held upright, tightly swaddled).
Then, years after his Dx, I found some old video tapes of him, and some of his younger brothers, when they were babies. I cried as I watched the videos, because the difference between my eldest and his younger brothers was so obvious.
With my eldest, the videos are full of me trying to elicit a response from him, saying his name over and over again, and exclaiming with joy if he looks at me. He is unresponsive and ignores me most of the time. With his brothers, even as very tiny babies, they are staring into my eyes as they feed, holding up their hands to my face, and touching it. They respond to my voice by looking, moving their mouths or hands and making noises. If I look away, they search for me.
I was so distracted by the more obvious signs of DS1's autism from the age of about 2.5 (horrific behaviour-screaming fits that would last for hours, throwing anything and everything, hitting, kicking, punching, shouting, scratching...), when DS2 was born, that I barely had time to glance at DS2, never mind notice the difference between him and how DS1 had been!
By the time DS3 came along, DS1 was 3.5, and he was full-on. I was so busy just surviving on a day-to-day basis that I noticed nothing!
The team who diagnosed DS1 kept asking about how he'd been as a baby, but I honestly couldn't see what they were getting at...until years later when I watched those videos, when I was no longer living in the thick of it! I couldn't believe I hadn't noticed how unresponsive and disinterested he had been!