Men and women refrain from cutting their hair during this period. This includes all facial hair and hair anywhere else on the body. Adults may not cut children’s hair. We are also restricted from trimming or waxing eyebrows, mustaches and the like, even hair around the private parts. If the mustache interferes with eating, it may be trimmed. However, it is permissible, indeed it is a mitzvah, to cut a forelock (tshup, in Yiddish) during the Three Weeks.
When a boy’s third birthday falls during the Three Weeks, the upshernish is delayed until the tenth of Av after midday. This however does not include the other customs associated with the upshernish, such as wearing tzitzit, saying the morning blessings, Birchat Hamazon and the bedtime Shema; these should begin on the day of the birthday.
Until Rosh Chodesh Av, it is permissible to cut and style a sheitel and it is not considered cutting hair. (From Rosh Chodesh Av, it is forbidden to wash a wig during the Nine Days. If there is a real need, there is room for leniency before the week of Tishah B’av. It is permitted to style and comb, even with help of hair products, and trim a wig (especially for reasons of modesty)).
Rav Yosef Yeshaya Braun, shlita, member of the Bet Din of Crown Heights; Halacha Corner, Moshiach Now! & “The Good in these Days” from Day-To-Day Halachic Guide; reprinted from crownheightsconnect.com
