There are two different types of traditions narrated in regards to the characteristic and general appearance of Imam Mahdi (as) as follows:…
In the name of Allah
Question 267: Hi assalamualikum, This about Imam al mahdi(as), that there is a video in detail a documentry on physical charecteristics of imam mahdi(as) by harun yahya,a turkish. He had explained well in details about how we can identify imam mahdi by looking at his physical charecteristics when he appear…its a long video and available on youtube so that i kindly request you to have a look on to get in details..( video titled : The holy appearance that will identify imam mahdi(as). By harun yahya.
The question is, How accurate are those information and are they authentic when compared to the shia point of view? Thanks
Brief Answer 267: According to some traditions, Imam Mahdi (ajtf) is the most similar person to the holy Prophet (PBUH). Some of his general appearance is as follows: youthful appearance with a brilliant face, curved eyebrows, a broad forehead, a nice stature, long straight nose, a small black spot on his right cheek, a face glowing like a shining star and had an appearance that no one has ever seen more beautiful, calm and pious complexion.
Answer 267: Imam Mahdī (ajtf) was born in 15 Sha‘bān 255 A.H. Since we are in the year 1438, the Imam (as) is said to be 1183 years old. After the death of His father (Imam Hasan al-Askari), Imam Mahdi (ajtf) were presented to offer prayer for the dead. All of the people that were presented there saw his face. He had a wheatish complexion, curly hair and a spacing with between his front teeth. 
There are two different types of traditions narrated in regards to the characteristic and general appearance of Imam Mahdi (as) as follows:
A: Traditions in which the Imam (as) is said to be youth without mentioning how old he is:
- He is young, though he is aged.
شابٌ بعد كبر السن
- He will appear at the best age of youth.
رجوعه من غیبته بشرخ الشباب
- He is very old but with youthful appearance.
فی سن الشیوخ و منظر الشباب
B: Traditions that rely on the both age and youthful appearance of Imam Mahdi as follows:
- with a complete youthful complexion and at the age of 30.
فی صورة فتیَ موفق ابن ثلاثین سنه
- Youthful complexion and less than 40 years old.
فی صورة شاب دون اربعین سنة
- He is old, but with youthful complexion in a way that people think he has less than 40 years old. One of his signs is that he won’t be aged till the time of death.
Sheikh Tusi quoted from those whom met the Imam (as) during the minor occultation that He was a handsome youth with fragrant smell and magnificent appearance. The narrator said: When he was delivering speech, we have never seen anyone can speak like him. 
It is narrated in other tradition that he is of medium height. He has a rounded head, broad forehead, long strait nose, shine cheeks and a small black spot on his right cheek.
It is quoted from Imam Ridha (as) that he has said: One of his signs is that he is aged but with a youth appearance that when anyone look at him he is seemed to be at the age of 30 or less. He won’t grow old until he will die.
When it comes to having long life, it is not specialized for Imam Mahdi (ajtf). Some individuals like the Prophet Noah (as) that lived long life (2500 years old), Prophet Khidhr (as) with thousands years old that he is still alive, Loqman bin Aad (3500 years old), Loqman Hakim or the Prophet Jesus (as) that he is still alive.
In order to justify it scientifically we can say that since getting old and death are because of some attrition factors. Therefore, if there are no attrition factors one would remain young.
Know that Imam of the time (as) is the very beautiful and good-looking person among people. Because, he is the most similar person to the holy Prophet (PBUH).
Jaber bin Abdullah Ansari narrated that the Holy Prophet (PBUH) has said: Mahid (as) is one my children. His name is my name, his Kunyah is my kunyah. He is the most similar person to me.
Ibrahim bin Mahziyar whom met Imam Mahdi (ajtf), quoted: The Imam (as) was young with a brilliant face, his eyebrows were curved, he had a broad forehead, a nice stature, long straight nose, a small black spot on his right cheek, a face glowing like a shining star and had an appearance that no one has ever seen more beautiful, calm and pious complexion like him.
Meeting with the Imam of Time (‘atf) during the period of occultation:
It is possible to meet with the Imam of Time (‘atf) during the period of occultation. Many scholars who is believed to be honest and just (‘adil) and who can be trusted in such matters have narrated stories of people meeting with Imam Mahdi, may Allah hasten his reappearance. These stories can be found in their books along with names of people who have been blessed with the opportunity to meet with him. Such meetings have taken place both during the period of minor occultation and major occultation.
It is inferred from narrations that the Imam of Age (May Allah hasten his reappearance) is not living far away from people and society. In fact, he is living with people and in their midst. He attends public congregations but people do not recognize him.
For further information in this regards, please refer to the following answers:
Index: Possibility of Establishing Relationship with Imam Mahdi, answer 513.
Index: The Signs of the Re-Emergence, according to ibn Mahziyar Ahwazi, answer 072.
Index: The Benefits of Imam Mahdi (ajtf) during His occultation, answer 026.
Index: When appear: Imam Mahdi (ajtf) brings a new order, Method or Shari’ah with Him, answer 022.
 . Refer to: Mawood Shenasi, Pg. 437.
 . Biharul Anwar, Vol. 51, Pg. 217, chapter 13 (ما فيه ع من سنن الأنبياء).
 . Gheibat Tusi, Pg. 421.
 . Ekmal al-Din, Vol. 2, Pg. 376.
 . Biharul Anwar, Vol. 52, Pg. 287, chapter 26 (يوم خروجه و ما يدل عليه).
 . Ekmal al-Din, Vol. 1, Pg. 316.
 . Ekmal al-Din, Vol. 2, Pg. 652; Ibid, chapter 57, Pg. 558, H. 12.
 . Refer to: Mawood Shenasi, Pg. 437.
 . Ibid.
 . Mawood Shenasi, Pg. 555.
 . Refer to: Edalat Gostar Jahan, Ibrahim Imani.
 . Haeri Qazwini, Mikyal al-Makarem, Vol. 1, Pg. 132.
 . Sheikh Sadouq, Kamal al-Din wa Tamam al-Ne’amat, translated by Mansoor Pahlawan, Vol. 1, chapter 25, Pgs. 534-535, H. 1.
 . Ibid, Vol. 2, chapter 43, Pgs. 180-181.