Can a person be excused for not praying because he is unaware that it is obligatory?

Firstly: 

Not praying is major kufr and the person who does not pray is
beyond the pale of Islam because of his not praying. This is indicated by
the Qur’aan and Sunnah and by the consensus of the Companions (may Allah be
pleased with them). 

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

“Every person is a pledge for what he has earned,

39. Except those on the Right (i.e. the pious true
believers of Islamic Monotheism).

40. In Gardens (Paradise) they will ask one another,

41. About Al‑Mujrimoon (polytheists, criminals,
disbelievers) (and they will say to them):

42. ‘What has caused you to enter Hell?’

43. They will say: ‘We were not of those who used to offer
the Salaah (prayers)’

[al-Muddaththir 74:38-43]

It was narrated that Jaabir ibn ‘Abd-Allaah (may Allah be
pleased with him) said: I heard the Messenger of Allah (blessings and peace
of Allah be upon him) say: “Between a man and shirk and kufr there stands
his giving up prayer.” Narrated by Muslim (82). 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him)
said: Anybody who is accountable and dies when he is not praying, is a
disbeliever; he should not be washed, the funeral prayer should not be
offered for him and he should not be buried in the Muslim graveyard; his
relatives do not inherit from him, rather his wealth belongs to the bayt
al-maal of the Muslims according to the more correct scholarly view, because
the Prophet (blessings and peace of Allah be upon him) said in this saheeh
hadeeth: “Between a man and shirk and kufr there stands his giving up
prayer.”. Narrated by Imam Muslim in his Saheeh; and because the
Prophet (blessings and peace of Allah be upon him) said: “The covenant that
stands between us and them is prayer; whoever does not pray has
disbelieved.” narrated by Imam Ahmad and the authors of al-Sunan with
a saheeh isnaad, from Buraydah (may Allaah be pleased with him). 

‘Abd-Allaah ibn Shaqeeq al-‘Aqeeli, the great Taabi’i (may
Allaah have mercy on him) said: The companions of the Prophet (blessings and
peace of Allah be upon him) did not think that failing to do any action was
kufr except prayer. And there are many hadeeth and reports with similar
meanings. 

This applies to the one who does not pray because he is lazy,
but he does not deny that it is obligatory. As for the one who denies that
it is obligatory, he is a disbeliever and an apostate from Islam according
to all the scholars. End quote. 

Fataawa al-Shaykh Ibn Baaz
(10/250). 

Secondly: 

If a person who is not praying dies, it makes no difference
whether he is in a Muslim country or a non-Muslim country, but his sin is
greater if he was living among Muslims, because he could see the people
praying and hear the call to prayer at all times. 

Thirdly: 

Some people may become Muslim and be living in a non-Muslim
state and not know anything about the pillars and duties of Islam, such as
prayer and so on. This may be imagined in the case of one who grew up in a
wilderness far away from knowledge and Muslims, or who lived in the jungle
and so on. Such a person is not judged to be a disbeliever or even a sinner,
because he is excused due to his ignorance. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said: 

But among people are some who are ignorant of some of these
rulings and may be excused for that, so no one should be judged to be a
disbeliever unless proof is established in his case that the message reached
him, as Allah says (interpretation of the meaning): “in order that
mankind should have no plea against Allaah after the (coming of) Messengers”
[al-Nisa’ 4:165]. Hence if a man became Muslim and did not know that
prayer was obligatory upon him, or he did not know that alcohol was haraam,
he does not become a disbeliever for not believing that the former is
obligatory or that the latter is haraam; indeed he will not be punished
unless proof from the Prophet reached him. 

Majmoo’ al-Fataawa (11/406) 

Ibn Hazm (may Allaah have mercy on him) said: 

There is no difference of opinion among the scholars
concerning the fact that if a man became Muslim and did not know the laws of
Islam, so he believed that alcohol was permissible and that a person did not
have to pray, and the ruling of Allah had not reached him, he is not a
disbeliever. And there is no difference of opinion concerning that, unless
proof was established to him and he persisted. In that case, according to
the consensus of the ummah, then he is a disbeliever. 

Al-Muhalla (11/206) 

With regard to his ignorance, it is stipulated that the
person should not have been able to dispel it by means of asking and seeking
knowledge. 

Al-Qaraafi al-Maaliki (may Allaah have mercy on him) said: 

The shar’i principle indicates that any kind of ignorance
which the accountable person could have dispelled does not count as evidence
for the ignorant, for Allah sent His Messengers to His creation with His
messages, and He commanded them all to learn it, then act upon it. So
learning and acting upon it are obligatory, and the one who does not learn
and act accordingly, and remains ignorant, has committed two sins because he
has failed to fulfil two obligations. 

Al-Furooq (4/264). 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
said:

This excuse does not count unless one is unable to remove it,
otherwise if a person is able to learn the truth but falls short in that, he
is not excused.

Majmoo’ al-Fataawa (20/280) 

Shaykh Muhammad al-Ameen al-Shanqeeti (may Allaah have mercy
on him) said: 

As for the one who is able to learn but he was careless about
it, and gives precedence to individual’s opinions over what he knows of the
revelation, this is the one who is not excused.

Adwa’ al-Bayaan (7/357) 

See also the answer to question number
10065. 

And Allah knows best.