The complexity of the process of foreign language (henceforth FL) learning highlights the seemingly infinite number of variables involved. It further entails that although most people can learn an FL and achieve a satisfactory degree of proficiency in both receptive and productive skills, no matter how old they are, there remain certain learners characteristics which to some degree determine the learners mastery of the FL. The learners age forms a part and parcel of these characteristics and has been the subject of much research over the last 40 years. It is, in fact, the most salient characteristic that makes the issue of the availability of what is called the critical period quite controversial among the specialists in language learning. It has paved the way to a further key issue and prime question in FL research and teaching, namely the differential success of children and adults in learning a new language.