Sunni scholars say that it is MANDATORY to follow one of the four sunni schools of thought, what happens to people who lived before the four Sunni Imams were born and reached the age of becoming scholars?

To my knowledge, all Sunni scholars believe that following a madhab is not a requirement for being considered Muslim. Thus those early generations are safe, and indeed are an inspiration to the schools that came after them. Even the founders of the four most well-known schools today were very specific that their rulings were the result of ijtihaad, which is an imperfect process. For example, Ahmad Ibn Hanbal said: "Do not follow my opinion; neither follow the opinion of Malik, nor Shaafi'i, nor Awzaa'i, nor Thawri, but take from where they took." So if someone follows the Qur'an and Sunnah, even according to the founders of the four schools, then they are Muslim and shall enter Jannah in sha Allah. Imam Ahamd's saying also points to the fact that there were more than four schools, e.g. the school of Sufyan Al-Thawri (http://en.wikipedia.org/wiki/Thawri). Four are most popular today, but this is mostly for historical reasons; there's nothing special about the four. However, some Muslim scholars, not all, argue that it is safest or most prudent to follow a madhab, for the following reasons: It is hard for many people who live busy lives to really understand the depths of Islam and come to conclusions on the more detailed parts of Islam (e.g. rulings on situations that rarely occur). Therefore it is better to follow a particular school or scholar who has studied these topics, rather than having to go directly to the Qur'an and Sunnah to answer these questions. Each school has analyzed a large number of day-to-day questions and come up with answers to questions that were not covered by the Qur'an and Sunnah explicitly. This creates a very useful source for rulings. Each school has a different methdology (Usool al Fiqh) as to its determination of rulings on subjects. If you "mix and match" between one madhab and others, you may end up with inconsistent rulings. So for these reasons, some Sunni scholars recommend/encourage following a madhab in the modern age for people who don't have enough time/knowledge to explore jurisprudential issues. Many Muslims (like myself) are Sunni, but do not follow a particular school; and are taught by highly respected Muslim scholars that also do not follow a particular school, but rather look at the opinions of past scholars (not just the four schools, but others as well) as a source of guidance. This is a much preferred arrangement to other strands of Islam, where one person -- who is fallible -- is falsely ascribed with some kind of divine infallibility to error. Then when they make a mistake in a ruling, it can lead to massive adverse consequences.

It is not mandatory. There is a difference between the concept of Imam in Sunni and Shia understanding. According to Sunni understanding the religion was completed during the life of Prophet Muhammad. So any Imam after that is a  teacher, you can agree with him or disagree with him. Whatever he says is his understanding you are not compelled to follow him.

Nobody says you to strictly follow one among four scholars then only you are on right path.... The only way to succeed is the way Prophet(PBUH) told us... And these Imams studied The QURAN & HADITH deeply and then collected the conclusions in the form of books for the ease of the peoples....books were not so common before... Now if you don't want to follow any of them or the way they are mentioning then its okay you study the whole Quran and all the Hadith and then you follow it... And the other way is read their compilation and follow (but dont leave the Salat because of not knowing how to perform it), and they are not telling the different thing they says that Prophet(PBUH) did it like this, or told us to do like this and so and so.... Now for the ease of people means that for example: if you want to learn the Salat then you will go for Hadith that mentions the way of performing Salat but you will not find whole Faraiz, Wajib and Sunnah in single hadith in fact you will have to go for many Hadith and aayahs of Quran....So they collected them and wrote the whole  Faraiz & Sunnah at single place... You have started following any one of them now Whats next? The answer is now keep on reading Hadith and Quran, in fact you must read it because they have their own importance and fazail... If you find something contradictory then consult Ulema... Always learn with good intention i.e, learn to follow. Ya Allah! Guide us to the right path. And forgive me if i made any mistake in writing. (Ameen)

1- A Muslim is either a Mujtahid or a Muqalid. A Mujtahid is the one who makes ijtihad and he is a high ranked scholar that can get islamic rulings from Quran and Sunna and Ijma^. There are many specs of the Mujtahid like knowledge in Ahkam al Quran and Ayat al Ahkam, the types of Ayat, the general, the specific, the Nasikh the Mansoukh in addition to hadith knowledge and so on. A Mujtahid is a very rare scholar with extraordinary abilities. A Muslim if not a Mujtahid must follow a  Mujtahid. A Muqalid is an imitator of the Mujtahid in the rulings that he made. The 4 imams are Mujtahideen, but there are many more that Muslims have followed from the day of the prophet until now. 2- There are around 70 years between when Muhammad died and Abu Hanifa was born. Abu Hanifa met with companions and with people that came after them tabi^een. So not much different in time here. 3- When the Prophet made the last hajj trip, the companions were about 100,000 of these scholars estimate around 200 of them were Mujtahideen. 4- The 4 imams are only famous because their schools got documented properly. Imam Awza^i for example is another famous Mujtahid who had his school just like the 4 imams. However, It was not documented and lost. So is Sufyan Athawri and many more. 5- People before following Abu Hanifa and after Abu Hanifa followed Mujtahids. Some documented and some did not. The most documented to date are the 4 famous schools and so people select one of them and follow them. It is easier and there is no need to look for Mujtahids these days as they are very rare. 6- There is misinformation in your question where Shia are assumed to have followed 1 Imam. No that is not the case. From day 1 there were no Shia. Imam Ali was a soldier in Khalid Bin Al Walid's army under Abu Bakr rule. All Muslims who were not Mujtahidin followed a Mujtahid as I mentioned. Some followed Ali some followed others. Ali was a Mujtahid. Shia came later on and they did not follow Ali as they claim or any or the imams.