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. 376
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. 551