“But winter babies are more likely to die before they turn one.”?

Question

The question is from the post here. I don't understand what does the last sentence mean here.

The Chillinghams can give birth all year, not just spring and summer like most U.K. mammals. And the data show that over the past few decades, there’s been an increase in winter births. Which the researchers correlated to warmer springs the previous year: plants grow earlier, the cattle have more access to nutrition and they conceive earlier.

But winter babies are more likely to die before they turn one.

Here is my question:

What does "turn one" mean? How can the "babies" turn to "one" baby?

Answer 1

The sentence means that winter babies are more likely to die before their first birthday (or before they become one year old).

Turn *number* in such contexts denotes age.

Another example would be "My goal is to retire by the time I turn thirty five", which means I would like to retire by the time I am thirty five years old.