Moed Katan 14B

ואי אמרת קטן אית ביה פלוגתא ⁠— נמצאת אבילות נוהגת בקטן.

And if you say that with regard to a baby there is a distinction between being born before the Festival and being born on it, and in some cases it is prohibited to cut the hair of a baby, then you find that mourning is practiced even with a minor.

והתניא: מקרעין לקטן, מפני עגמת נפש!

Isn’t it taught in a baraita: One rends the clothes of a minor whose relative has passed away, due to the desire to bring about feelings of grief among those who see him? However, there is no inherent requirement for the minor to observe any of the halakhot of mourning.

אמר רב אשי: מי קתני הא אסורין! דלמא יש מהן אסור ויש מהן מותר.

Rav Ashi said: Is it taught explicitly in the first baraita cited by Rav Pineḥas that if it is prohibited for one to have a haircut during the Festival, it is likewise prohibited to have a haircut during the week of mourning? That was merely an inference. Perhaps there are those among them, i.e., those listed in the Mishnah, for whom it is prohibited, while there are others among them for whom it is permitted, and the halakhot of mourning do not apply to a baby.

אמימר, ואי תימא רב שישא בריה דרב אידי, מתני הכי. אמר שמואל: קטן מותר לגלחו במועד, לא שנא נולד במועד ולא שנא נולד מעיקרא.

Ameimar, and some say it was Rav Sheisha, son of Rav Idi, taught that statement in this manner. Shmuel said: With regard to a baby, it is permitted to cut his hair during the Festival. It is no different whether he was born during the Festival, and it is no different whether he was born beforehand.

אמר רב פנחס, אף אנן נמי תנינא: כל אלו שאמרו מותר לגלח במועד ⁠— מותר לגלח בימי אבלו. הא אסורין לגלח במועד ⁠— אסורין לגלח בימי אבלו.

Rav Pineḥas said: We, too, learn in the baraita a support for this statement: All of those about whom the rabbis said: It is permitted to shave and cut his hair on the intermediate days of the Festival, it is also permitted to shave and cut his hair during the days of his mourning. But from this it follows that one who is prohibited from shaving and cutting his hair on the intermediate days of a Festival is also prohibited from shaving and cutting his hair during the days of his mourning.