Puberty is reached when one of three signs occurs in males, to which a fourth sign is added in the case of females.
These three signs are as follows:
- Reaching the age of fifteen years.
- Growth of hair in the pubic area (private parts) – for both men and women.
- Emission of maniy [sexual fluid] accompanied by feelings of desire, whether in a wet dream, during intercourse or through any other means.
In the case of women there is a fourth sign, which is the onset of menstrual bleeding.
If any of these signs appear, the person is judged to have reached puberty, so he becomes a responsible adult, required to obey all commandments and heed all prohibitions.
Source: https://islamqa.info/en/answers/7480/when-someone-dies-under-the-age-of-40-will-he-go-to-paradise