How does the time dimension affect income inequality?

Movement of individuals or households from one income quintile to another over time is called income mobility. For most income receivers, income starts at a relatively low level during youth, reaches a peak during middle age, and then declines. Thus, if all people receive exactly the same income stream over their lifetimes, considerable income inequality would still exist in any specific year because of age differences. In any single year, the young and the old would receive low incomes while the middle-aged receive high incomes. These facts about individual and family income mobility over time are significant; for many people, "low income" and "high income" are not permanent conditions. Also, the longer the time period considered, the more equal the distribution of income becomes.