Why oh why do people teach their kids upper case letters first? OK that was rhetorical. They are easier shapes, but please don’t think you are helppng your child. YES they will recognise the upper case letter their name starts with but the lower case letters will be more useful.
Try convincing a child that R is the same as r.
ShibbOleth. Early, in this context, is preschool. Age three is not too early to start (please note I said start) teaching a child to rote recite the alphabet, recognize upper and lowercase letters in their name, count to five or higher, identify numbers 0-9. By count, I mean one-to-one correspondence: actually seeing, say, ten items and correctly counting them. Start them counting their fingers and toes, then your fingers and toes.
I said, the disadvantages are a risk if the lagging persists. There are no significant disadvantages if a child does not learn to read until age five versus a child reading a year older. There are however, distinct advantages to getting your child in a gifted student track if there’s documented evidence from prechool, kindergarten and first grade of them excelling academically, and reading well above grade level is a key part of that.
I have no cites. This is all anecdotal evidence based on my coming from three generations of educators, incuding my five years teaching kindergarten.
Parents teach children uppercase letters because that’s what’s commonly sold in flashcards, refrigerator magnets and other home school kits.