After theo turned 13, he noticed that his voice deepened and the hair on his arms and legs grew thicker and darker. both of these physical changes are considered to be typical changes in boys' that occur during puberty.

The physical changes during puberty are due to secondary sex characteristics

Explanation:

Origin: Secondary sex characteristics in male typically originate at the onset and develop during puberty from 9-16 years.

Mechanism: The hypothalamus of the brain initiates the release of follicular stimulating and leutinizing hormones which trigger the Sertoli cells of the testes to begin spermatogenesis and the interstitial cells of Leydig to produce the male sex hormone testosterone.

It is this release of testosterone that leads to the development of secondary sexual characteristics like:

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