He fasted before he reached puberty and he forgot to make up some days. Can he make themup after reaching puberty?

When I started middle school – and I was 13 years old at that time – I
broke the fast on three days and forgot about it, and I did not
remember ituntil this year. Now I am sixteen and I have not reached
puberty. Should I make up the fasts only or do I have to do anything
else?.
Praise be to Allaah.
Firstly:
A child is not obliged to do the obligatory duties of Islam until
after he reaches puberty, because the Prophet (peace and blessings of
Allaah be upon him) said: "The Pen has been lifted from three: from
the insane person who has no control over his reason until he
recovers, from thesleeper until he wakes up,and from the child until
he reaches puberty." Narrated by Abu Dawood (4399); classed as saheeh
by al-Albaani in Saheeh Abi Dawood. Whatever the child does of
obligatory duties or mustahabb deeds before reaching puberty, he will
be rewarded for them. Whoever performs Hajj when he is a child or
fasts or prays, the reward for that will be written for him in full,
but his Hajj does not count as the obligatory Hajj or Islam, and he
has to do Hajj again after he grows up.
It was narrated from Ibn 'Abbaas (may Allaah be pleased with him) that
the Prophet (peace and blessings of Allaah be upon him) met some
riders in al-Rawha' and hesaid: Who are these people?" They said: We
are Muslims. They said: Who are you? He said: "The Messenger of
Allaah."A woman lifted up a child and said: Is Hajj valid for this
one? He said: "Yes, and you will have the reward." Narrated by Muslim
(1336).
Al-Nawawi (may Allaah have mercy on him) said:
In this report is the evidence for al-Shaafa'i, Maalik, Ahmad and the
majority of scholars, who said that the Hajj of a child counts and is
valid, and he will be rewarded for it, but it does not count as the
obligatory Hajj of Islam, rather it counts as a voluntary Hajj.This
hadeeth is clear on this issue. End quote.
Sharh al-Nawawi (9/99).
Al-Khattaabi said:
It counts as Hajj for him with regard to attaining the reward, but it
is not tobe counted as his obligatory Hajj if he lives until the age
of puberty and reaches manhood. This is like prayer, which he (the
minor) should be told to do if he is able to, although it is not
obligatory for him, but the reward will still be written for him, by
the bounty of Allaah, and it will also be written for the one who
tells him to pray and teaches him to pray.
See: 'Awn al-Ma'bood (5/110).
He is not enjoined to make up the fasts that he missed when he was a
child, and he is not to be enjoined to make up any prayers that he
missed.
Ibn Qudaamah (may Allaah have mercy on him) said:
As for the days that he missed (of Ramadaan) before reaching puberty,
he does not have to makethem up, whether he fasted them or not. This
isthe view of the majority of scholars. Al-Mughni (3/94).
Shaykh Muhammad ibn Saalih al-'Uthaymeen (may Allaah have mercy on him) said:
It is not obligatory for a child to fast, but his guardian should tell
him to fast so that he will get used to it. Fasting – for the child
who has not yet reached puberty – is Sunnah, and he will be rewarded
for fasting, but there is no sin on him if he does not fast. End
quote.
Fiqh al-'Ibaadaat (p. 186).
Based on this, you do not have to make up these three days that you
did not fast, because fasting was not obligatory for you. But if you
want to make them up there is nothing wrong with that. You should only
make up the fasts, one day for eachday missed, and you do not have to
do anything else.
Secondly:
You say in your question that you are now 16 yearsold and have not
reached puberty.
But you have in fact reached puberty now, because puberty is reached
when one of three things happens which in the case of the male are:
1- Emission of maniy (semen)
2- Growth of coarse hairaround the private part (pubic hair)
3- Reaching the age of fifteen.
In the case of the female, there is a fourth sign which is the onset
of menstruation.
When one of the signs is established, then puberty has been reached.
It is notessential for all of these signs to be present.
See also question no. 70425 .
As you have reached the age of sixteen, you are considered to be an
adult,so you should act on that basis. The time of childhood and not
being accountable has come to an end, and the angels record everything
that theadult does, good or bad.
"So whosoever does goodequal to the weight of an atom (or a small ant)
shallsee it.
8. And whosoever does evil equal to the weight of an atom (or a small
ant) shall see it"
[al-Zalzalah 99:7-8]
And Allaah knows best.