The establishment of oak trees is often a slow and difficult process. Hence, it is necessary to determine the characteristics that can lead to improving their regeneration. In this genus, seed size is highly variable both at the interspecific and intraspecific levels, and the effects of intrapopulation variability are not well understood, being even less so for Quercus faginea. In this study, the effects of the age of the mother tree, seed weight and the interaction between these two factors on seed germination, emergence and growth (biomass) were analysed. For this purpose, 16 trees—8 young and 8 old—were selected with the intent to cover the entire range of acorn weights produced in this population. Among the main results, it should be noted that: (1) in older trees, it is easier to find larger acorns; (2) the percentage and the speed of germination of the acorns of young trees is greater than that of old trees; (3) the percentage and the speed of seedling emergence of young trees is greater than that of old trees; and (4) cotyledon weight is the variable that most influences biomass, quite often in a positive way. Therefore, maintaining intrapopulation variability seems to be an approach that most favours the persistence of these populations.