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I just heard the following complaint from a comedienne.

Humans are the only animal that is completely useless for the first twenty five years of life.

Obviously this is just a joke but it is true that primates and especially humans do have profoundly long juvenile periods compared with most animals. It's got me wondering whether humans really do spend the most time in sexual immaturity.

Unfortunately I have found this question hard to search because Google thinks I'm asking about the longest gestation period (which seems to be a much more popular question). To be clear, I am not asking about the longest gestation period.

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As you indicate in your question, the average age of sexual maturity is probably the best way to approach this, since immaturity is usually how juveniles are defined. Age of puberty is also different in boys and girls (the same goes for many animals), and has also decreased in the 21 century. However, as an historical average for humans 15 years is probably fair, even though there is a lot of variance. That is high compared to most animals, but there are some that have similar or higher ages of maturation, e.g.:

  • Cicadas in the genus Magcicada, which can have a 17 year life cycle

  • Elephants reach sexual maturity at about an age of 8-15 years, but usually dont start to breed until they are at least 18-20 years (see e.g. elephantsforever.co.za and Association of Zoos and Aquariums). Males mature and start reproducing later than females, and in practice it is mostly older bulls that reproduce.

  • The Nile crocodile, which reach sexual maturity at at an age of 12-16 years (largely dependent of body size though).

  • Some species of Tortoise reach maturity at ages 13-16 years (e.g. Gopherus sp, see Germano, 1994), and, again, which is largely due to body size and growth rate. Captive bred individuals can mature more quickly.

As you can see, there is a difference between sexual maturity and reproductive age. To actually be able to reproduce, especially in some male mammals, you often need experience and size, which means that reproduction is delayed in practice. Whether this period from sexual maturity (with respect to producing mature eggs/sperm) to reproductive age should be defined as least partially as "adolescence" will affect how humans compare to other animals. In my examples, I have focused on sexual maturity (which usually has less variation between individuals).

For other examples, you might want to look at insects other than Cicadas that have delayed larval stages. This does for many woodliving beetles, where development times can be very variable, and under poor conditions take up to 20-30 years. This is usually not the norm though.

When searching for other examples, use the terms "age of maturation, "reproductive age", "sexual maturity" etc, to focus on sexual maturation and not gestation.

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This article provides general facts about Tuataras, one of which is:

Tuataras reproduce slowly. They take 10-20 years to reach sexual maturity. Males can mate every year, but females breed every two to five years. It takes the female between one and three years to provide eggs with yolk, and up to seven months to form the shell. Then it takes an additional 12 to 15 months from copulation to hatching, possibly the longest incubation rate of any reptile.

A male tuatara named Henry, living at the Southland Museum and Art Gallery, became a first-time father at the age of 111. He fathered 11 babies with a female named Mildred, believed to be in her seventies.

Those creatures are therefore one of the many that can be added to the list of the above answer.

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The Greenland shark (Somniosus microcephalus) has been reported to need at least 156 ± 22 years reach to sexual maturity1. This is also thought to be the vertebrate with the longest lifespan.

Reference

1: Nielsen, J., Hedeholm, R. B., Heinemeier, J., Bushnell, P. G., Christiansen, J. S., Olsen, J., ... & Steffensen, J. F. (2016). Eye lens radiocarbon reveals centuries of longevity in the Greenland shark (Somniosus microcephalus). Science, 353(6300), 702-704.

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