Wikipedia's article "Negiah" begins:
The term negiah (Hebrew: נגיעה), literally "touch," is the concept in Halakha that forbids or restricts physical contact with a member of the opposite sex (except for one's spouse, children, siblings, grandchildren, parents, and grandparents). A person who abides by this halakha is colloquially described as a shomer negiah ("one observant of negiah").
If I have a young female cousin, at what age does she become forbidden to touch? I'm a guy in my mid-twenties.
According to R' Yosef Karo and others, "negiah" means "any touching at all". According to the Rambam and others, "negiah" means "affectionate touching". Whichever opinion is correct, the prohibition will start once she reaches a certain age, or a certain developmental stage. When will it start?
Please cite sources.
For help choosing among the conflicting views, CYLOR.