This is a pretty controversial belief. Girls do start speaking sooner than boys but they only get a few months’ head start. Language skills are supposed to even out between the genders by the time they end preschool.
If a young boy is lagging quite a bit behind in his speech and language development, don’t assume that it’s perfectly normal “because he is a boy”. Boys are more vulnerable to developmental delays or disorders that affect their language, communication and speech skills (up to 5 times more likely, in fact). So, the next time someone tries to tell you that you’re being too anxious that your son is not talking as much as another (girl) child “because boys are slower”, listen to your instincts, read up on the research, and seek professional opinions instead. At least then, you know for sure and can get early supports in as soon as possible if it’s needed.