Onion is one of the major cash crops and helps improve the livelihoods of smallholder farmers. Although genetically and morphologically different, seedlings of all released onion varieties are transplanted at the ages ranging from 40 to 50 days, which contributes to the low level of productivity in Ethiopia. Therefore, a field experiment was conducted at the research site of Fogera National Research and Training Center during the 2019/20 irrigation season with the objective of identifying the optimum seedling age for improved onion productivity in Northwest Ethiopia. The treatments consisted of three varieties (Adama red, Bombay red, and Nasik red) and four seedling ages (30, 40, 50, and 60 days) of onion, which were laid down in 3 × 4 factorial arrangement using randomized complete block design with three replications. Growth and yield parameters of onion were collected based on the standard procedures and analyzed using SAS version 9.6. The results revealed that both variety and seedling age significantly ( P <   0.001 ) influenced plant height, leaf number, bulb diameter, fresh bulb weight, and marketable bulb yield, while their interaction effect did not influence these parameters. Days to maturity was significantly ( P < 0.001 ) affected by main as well as by the interaction effects of variety and seedling age. Younger seedling took longer time to mature, while older seedlings matured relatively early. The seedlings of Bombay red variety at 60 days age gave the earliest maturity days (105.33 days) compared to the other combinations. The highest plant height (50.18 cm), leaf number (12.00), bulb diameter (53.47 mm), bulb weight (84.39 g), and marketable bulb yield (29.97 t/ha) were recorded from Bombay red variety where the marketable yield recorded from Nasik red was statistically similar with that of Bombay red variety. Seedlings with 60 days old performed best in terms of all the tested growth and yield parameters including the highest marketability (32.03 t/ha). Accordingly, 60 days old seedlings and Bombay red and Nasik red varieties can be used to increase the productivity of onion in the study area and areas with similar agroecology.