The field experiment was conducted on onion cv. Agrifound White (Allium cepa L.) at Horticulture Instructional Farm, C. P. College of Agriculture, Sardarkrushinagar Dantiwada Agricultural University, Sardarkrushinagar during the year 2010-2011 to study the effect of age of seedlings on growth, yield and quality of onion. Different age of seedling did not exert significant effect on plant stand per plot. The plant height (29.85, 59.66 and 62.90 cm) at 45, 75 and 90 DATP, number of leaves per plant (5.14, 8.60 and 10.94) at 45, 75 and 90 DATP, neck thickness (1.37 cm) at harvesting time, bolting per cent (22.99%), diameter of bulb (5.07 cm), number of doubled bulb (22.42) per plot, weight of doubled bulb (2.64 kg) per plot, total yield (27.35 kg) per plot, marketable yield (19.68 kg) per plot, unmarketable yield (7.67 kg) per plot and Total Soluble Solids (14.47 %) were significantly higher with transplanting of 8 weeks seedling i.e. treatment S3. The minimum days required for maturity (147.00) was recorded with transplanting of 8 weeks seedling.