Firstly: 

It should be noted that it is possible for a person to see the Prophet (peace and blessings of Allaah be upon him) in a dream, and that the Shaytaan cannot appear in the image of the Prophet (peace and blessings of Allaah be upon him), but he could appear in another form and claim that he is the Prophet (peace and blessings of Allaah be upon him). 

It was narrated that Abu Hurayrah said: I heard the Prophet (peace and blessings of Allaah be upon him) say: “Whoever sees me in a dream will see me when he is awake; the Shaytaan cannot take my shape.” 

(Narrated by al-Bukhaari, 6592; Muslim, 2266). 

According to a report narrated by Ahmad (3400): The Shaytaan cannot resemble me.” 

Al-Haafiz Ibn Hajar said: 

We have narrated it with a complete isnaad from Ismaa’eel ibn Ishaaq al-Qaadi from Sulaymaan ibn Harb – who was one of the shaykhs of al-Bukhaari – from Hammaad ibn Zayd from Ayyoob who said: If a man told Muhammad (meaning Ibn Sireen) that he had seen the Prophet (peace and blessings of Allaah be upon him) [in a dream], he would say, “Describe to me the one whom you saw.” If he gave a description that he did not recognize, he would say, “You did not see him.” Its isnaad is saheeh, and I have found another report which corroborates it. Al-Haakim narrated via ‘Aasim ibn Kulayb (who said), my father told me: I said to Ibn ‘Abbaas, “I saw the Prophet (peace and blessings of Allaah be upon him) in a dream.” He said, “Describe him to me.” He said, “I mentioned al-Hasan ibn ‘Ali and said that he looked like him.” He said, “You did indeed see him.” Its isnaad is jayyid. 

Fath al-Baari, 12/383, 384. 

With regard to those who say that the Prophet (peace and blessings of Allaah be upon him) may appear in all forms [in dreams], quoting as evidence the report narrated by Ibn Abi ‘Aasim from Abu Hurayrah, that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever sees me in a dream has indeed seen me, for I may be shown in all forms” – this hadeeth is da’eef (weak). 

Al-Haafiz Ibn Hajar said: 

Its isnaad includes Saalih the freed slave of al-Taw’amah, who is da’eef (weak) who became confused (in old age), and this is a report from someone who heard it after he became confused. 

Fath al-Baari, 12/384. 

Secondly: 

What is mentioned in the question about having seen the Prophet (peace and blessings of Allaah be upon him) as a young man or a child is possible, but it is also subject to the conditions mentioned above, which is that he should look as he would have looked at that age. 

Al-Haafiz Ibn Hajar said:  

The phrase “[the Shaytaan] cannot” indicates that although Allaah gave him the ability to appear in any form he wants, He does not allow him to appear in the form of the Prophet (peace and blessings of Allaah be upon him). This was the opinion of a group who said concerning the hadeeth: What this refers to is when a person sees him as he really looked. 

Some of them restricted it even further and said: He must see him as he looked when he died, so he must even take into consideration the number of his white hairs which was no more than twenty. 

The correct view is that he may be seen at any age or in any condition, so long as he appears as he really looked at any time, whether as a youth, in the prime of manhood, in old age or any other time of his life. 

Fath al-Baari, 12/386. 

Thirdly: 

Once this is understood, it is possible for the Shaytaan to come to a person in his sleep and claim to be the Prophet (peace and blessings of Allaah be upon him), if he comes in a form other than the way in which Allaah formed him at all stages of his life. 

The fact that there was a voice like in old times, or very handsome men, or someone who said “Muhammad (peace and blessings of Allaah be upon him)” has nothing to do with seeing the Prophet (peace and blessings of Allaah be upon him) in his true form. 

And Allaah knows best.