To answer directly: The Kardashian family members were generally in their 20s to early 30s when Keeping Up with the Kardashians first aired in 2007, with the youngest Kardashian/Jenner sisters in the single digits or teens and the older siblings in their 20s. Here are the commonly cited ages at KUWTK’s premiere:

• Kim Kardashian: about 27
• Kourtney Kardashian: about 28
• Khloé Kardashian: about 23
• Rob Kardashian: about 20
• Kylie Jenner: about 10
• Kendall Jenner: about 11
• Additional family members (e.g., Kris and Caitlyn Jenner) were in their 50s and 60s, respectively, at the time.

Notes:

• The show later transitioned to The Kardashians on Hulu in 2022, but your question concerns the age of the family when the original series began in 2007. If you’d like, I can provide a year-by-year table showing how old each member was at the premiere date and how their ages progressed over the initial seasons.