Is it permissible for a mentally disabled boy to look at a veiled woman's face if she unveils it in front of him? Should she observe the Hijaab in his presence? Is it permissible for her to look at him? Is there a specific age for the disabled boy when he is allowed to see my face unveiled? I cannot tell his real age because he is a stranger to me?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If this boy has not yet reached the age discernment, then you are not obliged to observe the Hijab in front of him. Ibn Qudaamah said, "As for young boys who have not yet reached the age of discernment, women are not required to veil themselves in front of them." [Al-Mughni]

However, when a boy reaches adolescence—in other words, when he is nearly fifteen years old—you should wear the Hijab in front of him. Ahmad ibn Hajar Al-Haythami said:

The correct opinion is that an adolescent is a boy who nears the age of maturity. In other words, when he reaches the common age of reaching puberty among his people—i.e. about fifteen, not nine years old, and perhaps another age—then he is like a mature person, so a woman has to observe the Hijab in front of an adolescent just like she has to observe it in front of an insane mature man. [Tuhfat Al-Muhtaj]

There is no sin in looking at his face as long as you do not look at him with lust. ‘Aa’ishah said, "I remember the Prophet, sallallaahu ‘alayhi wa sallam, screening me with his upper garment while I was looking at Abyssinians who were playing in the courtyard of the mosque." [Al-Bukhari and Muslim] This wording is that of Al-Bukhari .

Allaah Knows best.