1. How many babies, teenagers, and adults are "larks" or "owls"?
Scientists call a person's tendency to feel more awake earlier or later in the day their chronotype. People who feel best early are sometimes called "larks" and people who feel best late are called "owls"[1].
A research group at the University of Tübingen in Germany compared 26,214 children and young adults, from newborns up to age 30, using the same kind of measurement for every age. About 70% of babies aged 0 to 1 were morning types and about 1% were evening types. At age 16, only 5% were morning types and 19% were evening types[1].
Two cautions. These people were not followed for years. The study compared different people at different ages, so it shows a pattern across ages rather than the life of any one person. It was also done in Germany, so the exact percentages may differ elsewhere[1].
Even so, something seems to change during childhood and the teen years. Is it only that teenagers stay up late on purpose?
2. Does the body clock really shift later during the teen years?
The German study found that the turn toward evenings starts early, in childhood, rather than suddenly at 13[1]. A review of research on adolescent sleep by Stephanie Crowley and colleagues uses the name "perfect storm" for teen sleep: a combination of changes in the body's sleep systems and pressures from school and daily life[3].
I could read only the summary of that review, not the full text. So this article does not give exact hours for how much later the clock moves. The amount may differ from person to person; the studies cited here describe groups, not you in particular.
3. What changes in the body so that bedtime moves later?
Sleepiness comes from two systems. One is a pressure that builds up the longer you are awake, a bit like water filling a bucket. The other is the body clock, which sends a "night is coming" signal at roughly the same time each day. (The bucket is only a way to picture it. Real bodies are more complicated.)
A 2009 review in the journal Developmental Neuroscience by Hagenauer and colleagues argues that both systems change during adolescence, and that these changes are linked to later sleep timing[2]. Evening light may also play a part, but I could not confirm how much from the summary. That helps explain why "just go to bed earlier" is hard advice for a teenager: the body is not asking for sleep yet.
The same review points out that this delay clashes with early school start times[2]. This is a summary of one paper, and it does not say how big each cause is.
4. Do other animals also get sleepy later when they become adolescents?
Perhaps. The same review looked at studies in people and in animals and concluded that a delay in sleep timing during puberty may be common across mammals, not just humans[2]. If so, the teen night-owl pattern is not only a result of phones, homework, or modern life. I could read only the summary, so I cannot say which animals were studied, and the claim should be treated as "likely," not settled.
5. Does the late body clock last forever?
It does not seem to. In the German data, the shift toward evenings stops and turns back at about 15.7 years in girls and 17.2 years in boys. After the end of adolescence, the researchers found no significant change in early adulthood, up to age 30[1]. These ages are averages for groups, not a deadline for you.
So the "owl" years look like a stage of growing up more than a permanent personality trait. Some adults stay owls and some stay larks, and that varies from person to person.
6. How could you look at your own sleep schedule?
Try a one-week notebook. Every morning, write the time you woke up and how awake you felt, from 1 to 5. Draw a simple graph of your wake-up times, with days across the bottom and clock times up the side. Do not shorten your sleep or force yourself up early for the experiment. You are only watching. You might notice whether weekends look different from school days.
If you are tired a lot, or sleep troubles are making your days hard, talk to a trusted adult, such as a parent or teacher, or to a doctor. A notebook cannot tell you what is going on in your body, and it is not medical advice.
To read further, look up the Randler study titled "From Lark to Owl" in Scientific Reports (2017). It is free to read online, and its graphs show the age pattern well.