Until the age of 12 I lived with my dad in Nigeria but then I moved to my mum's country (USA) while growing up I never wore a Hijab. I know we are ordered as females to wear a Hijab but since I was never raised to wear one, do I have to start now at the age of (18)?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If the situation is as you described, you should repent to Allah from what you committed in the past. You should also wear Hijaab immediately. Whoever repents, Allah forgives him. Allah Orders Muslim women to wear Hijaab; Allah Says (what means): {O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments. That is more suitable that they will be known and not be abused. And ever is Allah Forgiving and Merciful..} [Quran 33: 59]

As for taking it off, it is forbidden as Allah Says (what means): {And stay in your houses, and do not display yourselves like that of the times of ignorance, …} [Quran 33: 33]

The Prophet said that the women who expose their beauty will be among the people who will be severely punished on the Day of Judgement.

He said “Two are the types amongst the denizens of Hell, the one possessing whips like the tail of an ox and they flog people with their help. (The second one) the women who would be naked in spite of their being dressed, who are seduced (to wrong paths) and seduce others with their hair high like humps. These women would not get into Paradise and they would not perceive the odor of Paradise, although its fragrance can be perceived from such and such distance (from great distance).” [Muslim]

For more benefit on the evidence for Hijaab and its conditions, please refer to Fataawa 81554, 83765, 83702, 81494, 354259 and 83033.

Allah knows best.