I often wondered this. I ended up going with the age DS was able to actually sit down and read something, so when he was reading a novel level 1 reader without any help and was able to enjoy and talk about what he had read. Up until that point I considered him to be learning to read.
I often wondered the same with crawling, I considered DS to be crawling when he was on hands and knees and able to purposefully move across the room to get something, but later I noticed many people saying their kids were crawling when they were just dragging themselves along or when they got up on all fours and wiggled out a little progress. There doesn't seem to be a set definition for any of this stuff which makes it hard to understand what is being asked for precisely when we are asked for milestones. I tend to always go with the most stringent definition of everything, I don't need to say DS talked at 1 month because he made a word like sound, crawled at 3 months because he traveled across his crib, walked at 7 months because he let go once while cruising, read at 1 because he knew the words stop and exit (from signs), etc. DS read at 3, late 3 even. Until that time he was not able to put it all together and actually read something in a way that I consider to be reading.
Don't get me started on all of the videos friends post on facebook of their kids "reading" memorized books or, my favorite, those readers they give out in K that repeat the same 2 words and add one novel word in, say a color, written in the color the word represents (I like Green. I like Yellow. I like Blue. etc.) Sorry, not reading. This would probably not bother me so much if I could also have posted early reading videos of my son. But of course I couldn't. That would be bragging and showing off, something you can't do once your kid is too far outside of the realm of average.
But yeah, that is what I went with and I feel confident and truthful saying that is when DS read. But I also keep in mind that my definition is certainly not the one everyone uses, especially on social media.
