The reason why it is prescribed for Muslims to perform Hajj once in a lifetime
Question

Why should Muslims atleast visit mecca once in their life time?
Praise be to Allah.We Muslims feel honoured and proud to be the slaves of Allah, the One, the Self-Sufficient Master, Who begets not nor was He begotten, and there is none co-equal or comparable unto Him; He is our Lord and we have no Lord besides Him. Hence we respond to the commands of our Lord with the utmost humility and submit to His commands, for we know that He is the All-Wise and no wisdom is greater than His. We know that He is the Most-Merciful and there is none more merciful than Him, may He be glorified and praised. Hence we love Him in a way that demands that we obey His commands even if that is somewhat difficult for us. We feel proud, happy and content when we do that which He commands us to do. 
For if a human loves another person, he loves to serve him and that may make him happy. So what do you think about the Almighty Lord Who created us and grants us provision, and everything that we have is a blessing from Him? And His is the highest description. We owe everything to our Lord, so we must hasten to do all that He commands us to do, so that we might give back some small thanks for His great blessings. We can never thank Him enough, but by His grace Allah the Most Generous accepts our small efforts and rewards us for them greatly. 
For example, Hajj or pilgrimage. If a Muslim does Hajj in the manner required by his Lord, then Allah has promised to forgive him his sins and admit him to Paradise, on condition that he does not spoil this action by committing any major action that would invalidate it and earn the anger of Allah. 
One of the great mercies that Allah has bestowed upon this ummah (nation, i.e., the Muslims), is that Allah has made obedience to His commands and the commands of His Messenger (peace and blessings of Allah be upon him) subject to the condition that one be able to do them. So long as a person is able to do them, then it is obligatory for him to do what is required of him, otherwise he is not obliged to do so and he is excused. Allah says (interpretation of the meaning): 
“Allah burdens not a person beyond his scope”
[al-Baqarah 2:286] – i.e., He does not ask him to do more than he can bear. 
Concerning Hajj in particular, Allah says (interpretation of the meaning): 
“And Hajj (pilgrimage to Makkah) to the House (Ka‘bah) is a duty that mankind owes to Allah, those who can afford the expenses (for one’s conveyance, provision and residence)” [Aal ‘Imraan 3:97]
By His mercy He has enjoined this upon His slaves once in a lifetime, so that it will not be too difficult for them, but He urges those who are able to do Hajj and ‘Umrah more than once to do so. The Prophet (peace and blessings of Allah be upon him) said: “Keep on doing Hajj and ‘Umrah, for they take away poverty and sins as the bellows takes away the impurity from iron.” (Narrated by al-Nasaa’i, 2/4; this is a saheeh hadeeth as was stated by al-Albaani in al-Silsilat al-Saheehah, 1200). 
This great act of worship was prescribed by Allah so that we might praise Him and glorify Him, and thank Him for His great blessings and bounty. The purpose of Tawaaf around the Ka’bah is not simply to go around these stones! No, rather the reason is that Allah has commanded us to go around it seven times and we obey Allah by going around it seven times, no more and no less; rather we do what He has commanded us and we feel that we are His slaves, humbling ourselves before Him and praising Him and thanking Him for choosing us to be His slaves out of all of mankind who worship various gods, and may even worship themselves or their own desires. 
The same applies to all the rituals of Hajj, and indeed to all the acts of worship that Allah has prescribed for us. Praise be to Allah, Who has honoured us with this great religion. 
Moreover, your interest in asking about the Hajj at your young age indicates that you are keen to learn and find out. We advise you to learn more about Islam and to read about it, and to find out for yourself that it is the religion that suits the natural inclinations of man. This will set you on the path towards pleasing your Almighty Lord, Who created you and Who provides for you, and Who deserves that you should worship Him alone and none other. 
Perhaps you know that our prophet Muhammad (peace and blessings of Allah be upon him) told us that his fellow-Prophet ‘Eesa (Jesus – peace be upon him) will come down at the end of time and will perform pilgrimage to this House, and he will declare his belief in Allah alone (Tawheed). We believe that this will happen as the Prophet (peace and blessings of Allah be upon him) told us, just as we believe that the sun rises in the morning. The Prophet (peace and blessings of Allah be upon him) said: “By the One in Whose hand is my soul, the son of Maryam (the son of Mary, i.e. Jesus) will certainly pronounce the Talbiyah for Hajj or for ‘Umrah, or for both  in the valley of Rawha.” (Narrated by Muslim, 1252). The valley of Rawha’ is a place between Makkah and Madeenah. 
[Translator’s note: The Talbiyah is a prayer recited by pilgrims going to Makkah. It may be translated as follows: “Here I am at your service, O Allah, here I am. Here I am at Your Service, You have no partner, here I am. All praise and blessings are Yours, and all dominion. You have no partner.”] 
We ask Allah to open your heart to true guidance… Ameen.