Complex sentence: During the latter decades of the 20th century tattooing became a popular social practice worldwide, and many, if not most, young people today either have aspirations to have a tattoo somewhere on their body or already have one or more.
Split into simpler sentences: During the latter decades of the 20th century tattooing became a popular social practice worldwide. Many younger (and some older) people today either have aspirations to have a tattoo somewhere on their body, or already have one or more.