I want to know if speaking to a boy younger than me is unlawful (haram) or not?

The answer depends on the boy's age and the nature of the conversation. There is no harm in conversing with a boy who has not reached puberty. However, if he has reached puberty, conversation between young men and women often leads to temptation, so a Muslim should be wary of Shaytan's embellishments.

The jurists of the four madhhabs have ruled that it is not permissible to speak with a young non-mahram woman without necessity, due to the strong likelihood of temptation. While an old woman may be verbally responded to, it is disliked to return greetings to and from a young woman. The Hanafis stated that the greeting should be returned inwardly.

Al-Nawawi said: If a man greets a woman, or a woman greets a man, then if there is a mahram relationship, or marriage, or she is his slave, it is recommended and obligatory to return the greeting; otherwise, no, unless she is an old woman in whom there is no fear of temptation. The Shafi'is disliked a young woman returning a man's greeting, and if she greets him, it is disliked to return it to her. However, if there are a group of women, and a man greets them, or vice versa, it is recommended if there is no fear of temptation, based on the narrated hadiths.

Therefore, it is advisable to avoid speaking with young men unless there is a legitimate necessity according to Sharia, such as buying and selling, provided there is no suspicion.