In modern broccoli production, it is important to know the optimal development and age of seedlings, due to the effect on early maturity and the components of yield and quality. Since the growth of seedlings depends on genotype, age and growing conditions, the aim of this study was to determine the growth and quality dynamics of broccoli seedlings depending on cultivar ('Lucky' and 'Batavia') and age (2, 3, 4, 5 and 6 weeks) during the spring growing season in a heated greenhouse. Sowing was carried out in four weeks of the cycle, in the period from 4 to 25 February 2020, in polystyrene containers with 209 pots with a volume of 21 cm³. To determine the growth dynamics of the seedlings, plant weight, stem height, number of leaves and root length were analyzed once a week on 15 plants per treatment two weeks after the first sowing. The effect of cultivar was noted only at the beginning of the study, when the cultivar 'Lucky' had significantly higher plant weight, stem height and root length, while the effect of age on these indicators was recorded throughout the study, with the oldest seedlings having significantly higher values. Throughout the experimental period, the factors studied had a minor effect on the number of leaves of the seedlings as well as their interaction on all the indicators observed. The higher relative growth rate (RGR) of seedling mass and stem length found in both cultivars between 2 and 3 weeks of age indicated faster growth of the aboveground part of younger plants, while the higher RGR of root length between 4 and 5 weeks of age indicated faster growth of underground part in older plants.