From the Canonlaw.info blog:
"Present liturgical law is clear that only adult males (viri) may have their feet washed at the Mass of the Last Supper: “Lotio pedum …11. Viri selecti deducuntur a ministris ad sedilia loco apto parata. Tunc sacerdos … accedit ad singulos, eisque fundit aquam super pedes et abstergit …” (Mass of the Lord's Supper, Roman Missal 2002). Therefore, if someone is washing the feet of any females (or, it seems, even of males under 18, per 1983 CIC 97), he is in violation of the Holy Thursday rubrics."