A “rice nursling seedling” is the youngest stage of rice seedling transplanted by machine in Japan, and is defined as a seedling of 2 to 3 plant age in leaf number. However, changes in morphological and physiological characteristics of the seedling during this stage of development are not fully understood. This study was undertaken to obtain the ideal plant age of rice nursling seedlings which would give excellent seedling establishment. When plant age of rice seedlings was approximately 2.4, substantial photosynthesis occurred and plant length was over 7 cm, which is required for mechanical transplanting. The transplanted nursling seedlings with this age developed depending on the emerging crown roots from the first node, and it was suggested that crown roots from the coleoptilar node played a role in the absorption of water and nutrients just after transplanting. When rice nursling seedlings exceeded the age of 2.6, characters of the seedling degraded, and this was aggravated by high-density seeding. Rice seedlings with plant age of 2.5 showed the highest level of submergence tolerance among rice seedlings with several plant ages. The results suggest that plant ages of 2.3-2.5 are the ideal ages for transplantation of rice nursling seedlings.