Reaching the age of accountability (i.e., puberty) is attained when one of three signs occur in the case of males; in the case of females there is a fourth sign, which is menstruation.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Puberty is reached with the occurrence of one of three things in males, namely:
1. Reaching the age of fifteen years
2. Growth of pubic hair
3. Emission of maniy with desire when awake or asleep.
If one of these three things occurs, then the individual has reached puberty.
In the case of females, a fourth sign is added to this list, namely menstruation. If a female menstruates, even if she is ten years old, then she has reached puberty.
End quote from ash-Sharh al-Mumti‘, 4/224
If a boy or girl emits maniy, accompanied by feelings of pleasure, whether when awake or asleep, then we rule that he or she has reached puberty, because of the evidence that we shall quote below from the Qur’an and Sunnah, and scholarly consensus.
Al-Maawardi (may Allah have mercy on him) said: With regard to wet dreams, that is regarded as a sign of reaching puberty or sexual maturity because Allah, may He be exalted, says (interpretation of the meaning):
“And when the children among you reach sexual maturity, then let them (also) ask for permission…”
And it was narrated from the Prophet (blessings and peace of Allah be upon him) that he said: “The Pen has been lifted from three” among whom he mentioned “the child, until he reaches sexual maturity.”
The definition of sexual maturity (referred to in the Qur’an and hadeeth) is the emission of maniy that gushes from a man or woman, when sleeping, when having intercourse or otherwise.
End quote from al-Haawi, 6/343; al-Mughni, 4/297
Thus it is known that emission of madhiy does not mean that the individual has reached puberty, because there is no evidence to that effect, and the basic principle is that the individual is not accountable until it is proven that there is evidence of change.
It says in Mataalib Ooli an-Nuha (3/404), after mentioning the signs of puberty, including the emission of maniy: … Puberty is not reached by anything other than what has been mentioned. End quote.
The growth of coarse hair around the private part, i.e., the penis in males and the vagina in females, is one of the signs of puberty, because of the report narrated by ‘Atiyah al-Qurazi (may Allah be pleased with him) who said: We were presented to the Prophet (blessings and peace of Allah be upon him) on the day of Qurayzah; those who had developed pubic hair were executed and those who had not developed pubic hair were released. I was one of those who had not developed pubic hair, so I was released. Narrated by at-Tirmidhi, 1584; classed as saheeh by Shaykh al-Albaani.
Ibn Qudaamah (may Allah have mercy on him) said: With regard to the growth of pubic hair, this refers to the growth of coarse hair around the penis of the male and the vagina of the female, which should be removed by shaving. As for fine hair, it is of no significance because it grows in the case of a small child.
End quote from al-Mughni, 9/393.
For more information please see the answer to question no. 138738
With regard to hair other than pubic hair, such as the hair of the beard or moustache or armpit hair, it is of no significance, even if it is abundant, because Islam only connected the ruling to the growth of pubic hair. If hair other than pubic hair was of any significance, the Prophet (blessings and peace of Allah be upon him) would not have ordered the exposure of the private parts of Banu Qurayzah, because the appearance of hair in places other than the private area would have been sufficient.
It says in Mataalib Ooli an-Nuha, 3/404: There is no other sign that could indicate that puberty has been reached, apart from what has been mentioned (so deepening of the voice, growth of the nose, development of the breasts, armpit hair, growth of the beard, and so on, are not signs of reaching puberty). End quote.
And Allah knows best.