Kids can tell left from right as early as 3 years. Every kid has a bank of words in their brain. For every kid the bank is a different size. Some kids need to hear a word 700 hundred times before they “know” the meaning of that word and can use it. Some kids with learning difficulties need lots more. Keep saying it to add to your child’s bank.
Never, never, never teach right with left at the same time. This is the mistake that creates adults with difficulties with left and right or difficulties recognizing b and d. Never teach them together. Pick one (if you kid is right handed, teach right, if he is left handed only teach left) and make sure that for him there is the right hand and “the other hand” for a long time before you introduce the left.
I am a special education teacher. You won’t believe how many problems there are only because similar things were introduced together and the kids (and grown ups) can’t tell which one is which.
So the age is not the important thing, but the way they learn it.