There are a few main reasons why we get busier as we get older:
Adulting
As you age, you increasingly lose free time towards dealing with “adulting” type of tasks: taxes, paying bills, taking your car to the shop, researching insurance alternatives
Relationships
We as age, we accumulate relationships. And while they have numerous benefits and make life worth living, they don’t come for free. They require time and energy to maintain — and at the end of the day, become tasks on our todo lists. Even something as innocuous as an old friend reaching to send a text or schedule a call can, at times, feel like burdensome tasks to accomplish.
When you’re a child or teenager, the only people you know are your family and your friends (your first generation of friends). Since you barely know anyone, you don’t really have to keep in touch with anyone. Thus, more free time.
Hobbies
We as age, we accumulate interests, hobbies, and pursuits. These also don’t come for free.
As an adult, you begin to explore the world — reading books, picking up rock climbing, learning to paint, planning & taking trips one or twice a year. Your old interests don’t exactly go away, and there are always worlds of new interests to discover. Part of you feels like you should maintain or get back to some of those old interests you cherished so much. Another part is excited to get into scuba diving.
When you’re a child or teenager, you you might have just one or two pursuits that occupy your time outside school. That lack of all the historical hobbies from your past = more free time.
Aging
Aging implies that your body will start to perform worse and more slowly, likely even breaking in ways. You’ll spend more and more time going to doctor’s appointments, surgeries, tending to medical conditions. It will take more effort to maintain your body through fitness. This all takes time.
Like many problems in life, the frustration at your seemingly decreasing time as one ages can be helped by setting expectations properly. Instead of feeling cheated as you feel your allowance of time seems to shrink year by year, expect it. Expect that by all logic, given the adulting to do, relationships to maintain, pursuits to keep up with, and the natural course of aging, you should have no free time at all — which gives you more reason to celebrate and appreciate the rare free moment when it comes along.