In general, the relationship between age and the amount of money that a person earns is not linear. That is, it does not start low and get higher for a person’s entire life. Instead, there is more of a curve in which a person starts out making very little money, gets to make more and more, and then at some point starts to earn less. To some degree, age is really not a causal factor, though.
Younger adults typically make less money than older adults. However, this is not so much because of their age per se. Instead, it is because of their experience and skills. Having not gained much experience or many skills, they are not yet as valuable as they could be to a firm. As they grow older, they make more money. Again, however, this is not really because of their age but more because of their experience and skills.
Of course, the trend of making more and more money does not typically continue. At some point, most people retire. It may be voluntary or they may no longer be able to perform the job they once did (for example, airline pilots face mandatory retirement due to age). At that point, a person’s income tends to decline.
Thus, one’s ability to earn tends to rise for most of one’s life, but it then starts to decline after a certain age.