It is not uncommon for a child to have trouble with gender words–he, she, his, her, etc.–for quite some time. My three and a half year old is almost getting it, but not my two year old. And my almost-five year old has it, I think completely, and he was linguistically challenged.
So I would say that some time between the third and fourth year is common for the gender thing to click, but allow a little longer without worry.
The first step in mastering gender word confusion is getting “girl” and “boy” down pat. If a child is remotely fuzzy on recognizing boys and girls, they’re not ready for “he” and “she.” So start with magazines and flip pages, asking your preschooler “Boy or girl?” Keep it simple: don’t do “man,” “lady,” “baby,” etc… just gender. Magazines are good because you get different ages, ethnicities, contexts, contortions, and facial expressions (which, believe it or not, can really affect recognition). You also get different sizes which can also really affect their recognition.
Note: While it is fine for preschoolers to know “real” differences between girls and boys, like private parts or having babies, this won’t help in their everyday recognition =) Facial and superficial recognition is what you need to master gender words. So go with the old politically incorrect helpers like: girls have long hair, pink, dolls, flowers, dresses; boys have short hair, blue, tools, bikes, etc. Whatever will help your child visually and quickly assign people to one category or the other. As they age, you can work off this foundation to alter perceptions as necessary. If you’re sensitive to this, try to think big picture. At 3 and 4, you aren’t giving them political advice. You are trying to hone their abilities to get information from some features of a picture and exclude others (i.e. skin type, wearing glasses, etc. tell you nothing about gender).
Once they get boy/girl down correctly from pictures, practice with real people to make sure. There are often fewer signs in real life than in a picture. They’ll probably enjoy repeating that Daddy is a boy and Mommy is a girl. If they’re having trouble with gender discrimination, you can also make a big deal about “boy” things like going to a barber or “girl” things like wearing nail polish…again, for education not politics. You want them to learn that there are gender “cues” in life, which supply information and help for things like recognizing what bathroom to use or what part of the clothing department you’re in.
When you’re reasonably sure they’ve got boy/girl, you can move to he/she. Often times a child says whatever pronoun they’ve heard most—either he or she—for both genders. As you teach them that boys are a “he” and girls are a “she,” they may switch and call everyone by the other pronoun. That’s ok. They’re just working it into their vocabulary. Just keep practicing and correcting as they make mistakes. “Is Sara a girl or a boy?… Girl, that’s right. So is she a “he or a “she”?… “She,” that’s right.” (Don’t worry that you gave them a clue, they probably didn’t notice). You can now go through the magazines and ask, “He or she?” Keep working, this is harder because they have to do two steps: identify boy/girl, then decide he/she. It is a good cognitive skill. Eventually it will become connected and natural. They may still mess up in speech though, so keep correcting as necessary. You don’t have to do it all the time, just sometimes. And every now and then, practice. “Is Mommy a “she” or a “he?” What about Sara?”
When your preschooler can do he/she, they may pick up his/hers (him/her) naturally. Just because they hear you saying he… his and she…hers in sentences enough to make the link. But if not, you can work on these pronouns the same way. “David’s a boy, right? And a he? Ok, so is this “his” or “hers?” Get the good ol’ magazines again if necessary. Or ask them to think of family friends. The same for “Mr.” and “Mrs.”