Venkatesh Rao compares the evolutions of the coffee and consulting markets:

Both coffee and consulting are products for minds. Psychoactive products that link to producer and consumer personalities. You should expect to see echoes of generational personalities. Both are also mid-career markets. Your coffee tastes settle down around when you settle down into an adult career.

You typically land in indie consulting in mid-career too. By age 30-35, you know how you like your coffee, and have enough work experience to be fuel a consulting career.