If any of the signs of puberty appear in a boy, he becomes accountable

Firstly: 

Puberty is reached when one of three
things happens in the case of males: 

1.Wet dreams

2.Growth of coarse
hair in the pubic area

3.Reaching the age
of fifteen

If one of these three things occurs, then
the individual has reached puberty. In the case of females there is a fourth
sign, namely the onset of menstruation. 

Please see the answers to questions no.
20475, 138738 and
192334 

Secondly: 

With the onset of puberty, the individual
becomes subject to shar‘i rulings that apply to him, so he should not shake
hands with women or look at them, and he should not be alone with a woman,
because he has now become accountable and the things that are prohibited for
adult men are also prohibited for him, because he is one of them. It is not
permissible to be careless about this matter, and it is essential to take it
seriously. So you must refuse to go to the women’s prayer hall, and refuse
to shake hands with non-mahram women or look at them, including your
cousins, even if they object to you doing that, because there can be no
obedience to any created being if it involves disobedience towards the
Creator. 

You can seek the help of your sister, for
example, or other mahram family members to explain the shar‘i teachings to
your cousins. As you are taking this matter seriously, they will have to
understand. 

Similarly, if your father or mother tells
you to go in to where the women are, or to shake hands with them, it is not
permissible for you to obey them; rather obedience is only with regard to
that which is right and proper, but if it involves sin then there is no
obedience to anyone. 

There is nothing wrong with you explaining
to them that you have reached the age of puberty and are now a man, so the
same things are haraam for you as are haraam for men. But this explanation
should be given politely and gently. 

You could also seek the help of one of
your relatives in that regard. 

Please see the answers to questions no.
45905 and 104165 

Fourthly: 

If a fourteen-year-old boy has not yet
reached puberty, he is very close to it, and such a person should not be
allowed to see women’s ‘awrahs. Women have to understand that and observe
hijab in the presence of one who is very close to puberty. 

Allah, may He be exalted, says
(interpretation of the meaning):

“and not to show off their adornment
except only that which is apparent (like palms of hands or one eye or both
eyes for necessity to see the way, or outer dress like veil, gloves,
head-cover, apron, etc.), and to draw their veils all over Juyubihinna (i.e.
their bodies, faces, necks and bosoms, etc.) and not to reveal their
adornment except to their husbands, their fathers, their husbands fathers,
their sons, their husbands sons, their brothers or their brothers sons, or
their sisters sons, or their (Muslim) women (i.e. their sisters in Islam),
or the (female) slaves whom their right hands possess, or old male servants
who lack vigour, or small children who have no sense of the shame of sex”

[an-Noor 24:31]. 

Ibn Katheer (may Allah have mercy on him)
said: “or small children who have no sense of the shame of sex” that
is, because they are so young, they do not understand anything about women
and their ‘awrahs, such as their soft speech or alluring ways of walking and
moving. If the child is small and does not understand such things, then
there is nothing wrong with him entering upon women. But if he is adolescent
or close to adolescence, such that he understands these things and can
distinguish between who is unattractive and who is beautiful, then he should
not be allowed to enter upon women.

End quote from Tafseer Ibn Katheer
(6/45-46) 

It was said to Imam Ahmad (may Allah have
mercy on him): When should a woman cover her head in front of a boy? He
said: When he reaches the age of ten years.

End quote from al-Mughni (7/100) 

With regard to reaching the age of fifteen
years, what is meant is that the boy has completed fourteen years and has
entered his fifteenth year. This is often applicable to one who says that he
is fourteen years old; he has completed fourteen years. Therefore this sign
of puberty is applicable in his case. 

Moreover, the years that count in this
matter are Hijri years, not Gregorian. This means that his age will be less
than if it is counted by Gregorian years. 

The guideline on puberty in your case is
that you have reached this age, which will help you to avoid embarrassment
when speaking to your family or your relatives, because there is nothing
embarrassing about reaching a certain age. 

Your cousins have to understand the shar‘i
ruling. It is not permissible for them to uncover in front of you or to
shake hands with you, because you have reached puberty. By the same token
you must not obey them by disobeying Allah with regard to shaking hands with
them or going to the women’s prayer hall. 

Your father has to understand that, and
you have to be firm about this matter, for Allah is not too shy to speak the
truth. There is nothing wrong with speaking frankly when necessary about
puberty, and explaining that it is not permissible for you to shake hands
with women or enter upon them. 

And Allah knows best.