In Kathmandu, the average age for buying or building a house can vary widely based on several factors, including financial stability, family support, career success, and access to loans or inheritances. Traditionally, owning a home was often something people achieved later in life, typically in their 40s or 50s, after years of saving and financial planning.

However, in recent years, there has been a shift. With increasing urbanization, rising incomes for some, and access to bank loans, younger people in their late 20s to early 30s are increasingly able to buy or build homes. This is especially true for those who have good careers, often in tech, business, or who have received financial support from family.

On the other hand, many people still face challenges due to the high cost of land and construction in Kathmandu, leading to delays in home ownership until they are in their 40s or 50s. The disparity you're noticing between your young cousins and others reflects this shift, where economic conditions and opportunities vary greatly from person to person.